The trouble with alignment
One of the best ways to spark a blazing row when it comes to D&D is to start discussing what exactly each of the alignments means. It can start innocently, by arbitrarily attempting to assign a D&D alignment to a character from outside the franchise. Before you know it, it's escalated into full-on warfare. Drinks thrown, people dying on the hill of “yes there was that time that they did this”, before a surly agreement to disagree leading into a period of cold warfare where you daren't mention any of the fandoms that were brought into the discussion because it will just reopen old wounds and oh, it's just too painful, make it stop...
The problem is, the alignment system breaks down what is actually a very complicated and nuanced thing – morality – into a 9 square grid, with no grey areas in between. It does this because D&D is a game, the idea is that you don't have to have a degree in philosophy to properly enjoy it, and there probably isn't space for all of the different flavours of good, evil, and in between that actually exist in the world on one rulebook-sized page. On top of that, everyone involved in the game will have their own perception of what constitutes, good, evil, neutral, chaotic and lawful, usually influenced by how good, evil, neutral, chaotic or lawful they are themself. This is how disagreements happen so easily – and trust me, if you haven't been involved in, or witnessed one of these yet then you're lucky. And brace yourself, because it's coming.
What is alignment?
If you've never come across this before, alignment is a combination of two choices during character creation that describes your character's general motivation. Each choice is a sliding scale with 3 different settings.
First, you choose whether you're good, neutral or evil.
Second, you choose whether you're lawful, neutral or chaotic.
Are you playing the most devout of clerics who unselfishly and unfailingly follows the strict rules of their order to protect and care for the innocents who are weaker and meeker than them? Pick Lawful Good. Are you playing a bloodthirsty maniac who gallops around spilling the blood of everything that they see that has blood, just because it's spillable but... actually no, wait, that emu has nice feathers, it can keep its blood? Pick Chaotic Evil. Do you feel totally neutral about being neutral? Pick Neutral Neutral – or True Neutral as it's called, because someone realised that Neutral Neutral was not good enough to be named twice.
What's the problem with alignment in D&D?
Aside from players having the odd difference of opinion, actual, serious problems can be caused when a player chooses an alignment that they think is the right one for their character (usually in good faith) and then get accused of acting “against their alignment”. This also comes in shades of grey (“but it's what my character would do!” is sometimes a legit response to this, even though that phrase is usually associated with someone deliberately being a dick). It's important to remember that there are players who don't live or die by nine choices in character creation, and they pick what they think is the closest without having the benefit of knowing anyone else's preconceptions.
So why is that an issue? Well, let's say for example that you did go with Lawful Good and you (shock horror) kill an orc that's come marauding into a retirement village full of elderly people who were just minding their own business. You've watched said orc bludgeon several of the village's unfortunate denizens to death, except for that flimsy-looking citizen over there, they have a nice hat, so no bludgeoning. All of the crimson gore that's being produced is slowly contributing to a nice mural to Gruumsh on the side of the village hall – it depicts an Orcish deity showing the middle finger to a Cleric that looks not unlike yourself. And so, filled with righteous fury, you mace the smug, monstrous bastard to death. You feel pretty good about yourself, like your XP bar just filled up a bit, and like you just stopped the senseless, wanton murder of the defenceless plebs who now surround you, showering you with gratitude.
Except someone at the table has a face like thunder. It's the DM. Why is he looking at you like you just threw your coffee all over his Dungeon Master's Guide? Because you broke alignment. You killed something. You're Lawful Good, the paragon of all that is pure in the world, and there, at your feet, is an orc that you murdered in cold blood, so shame on you, and this act will not go unpunished by your deity, the DM himself and, you know what? By the with the nice hat. They're now angrily attacking you with it. Lose 1 HP.
Clearly that whole situation is ridiculous, and I've just proved that I like to exaggerate... but this does actually happen. People roll Lawful Good characters and their Dungeon Master then expects them to go through entire campaigns without killing anything. Facing down a horde of goblins? Well I hope you brought plenty of extra rope, because after you humanely knock them unconscious (with your Fireball spell somehow) you're going to need to tie them up so they can think about what they've done before they then get released and rehabilitated back into the world as community service workers.
OK, what about Paladins?
If you're new to D&D you might not know yet that Paladins kind of have mandatory roleplaying requirements, where they must adhere to the tenets of their deity, or they are cast out and become Oathbreakers. Some people think this is very cool, though, and deliberately break their oath so they can have dark, edgy powers. However, now that we're in 5th edition, Paladins aren't tied to the alignment system by this feature any longer – instead they each have an entirely separate code that details how they should behave. This comes with wiggle room on how you align your character.
Can't we just do away with it?
In theory, you could just not use the alignment system at all in a game. Unfortunately, it's been released into the big wide world, so most people are already aware of it. It also comes with some handy benefits. For example, Adventurer's League rules literally ban Neutral Evil and Chaotic Evil characters from being used in those campaigns. This is a very good thing because it prevents certain types of player behaviour, such as stealing from other party members or ruining adventures by murdering everything in sight - including important NPCs. It also allows the authors of the adventure to take advantage of the good vs evil dynamic – one wants to vanquish the other, and if the heroes are a mixture of good and evil then it makes it very difficult for the entire party to naturally want to take part in the same adventure.
Another benefit is that it does bring some roleplaying opportunities along with it, in that it encourages people to think about how their character would act in a given situation compared to how they, the player, would act themself. You might feel like you want to rush in and save a unicorn from a minotaur, because you're a fan of unicorns, and it seems like the right thing to do but when you made your character, you wanted to roll up a hardened mercenary who only involves themself in fights when the money is good. The unicorn's coin purse is empty. It's shit out of luck. You're sad when the unicorn dies, but your character isn't, and that makes you feel better because you roleplayed them well.
What's the solution?
The number one social contract when playing any game with a group of real live people is “don't be a dick”. If you have a Lawful Good player running around robbing entire towns blind, then it's probably time for the DM to consider in-game consequences (after a warning that they're not really playing their character the right way). However, those in-game consequences don't have to be the tarrasque bursting out of the ground directly in front of them, or a deity striking them down, or, worse, they have to skip out levelling up or something crazy.
The point of the alignment system is that it's fluid. In the case above, the in-game consequences absolutely should be applied (maybe their deity doesn't listen to them any more if they were religious, or maybe it's as simple as they lose the trust of the town, or their party, or become wanted by the town guard). Rather than coming down like a ton of bricks, though, and forcing the player to continue as Lawful Good, ask if there is method to the player's madness. If there is, discuss changing their alignment. It sounds like they've become Chaotic. What do they intend to do with what they've stolen? Will it be going to the poor and needy? Maybe they're still Good if so. Will it be going into their own personal coffers? Sounds like maybe they're Neutral... depending on what they intend to do with it, in which case maybe something extreme has happened and they've gone Evil.
It's definitely worth saying that expecting characters to behave a certain way is a good thing in certain situations – in the above example, if there is no reason at all for the Lawful Good character to develop an acute case of kleptomania, there will probably be a case for a conversation about how the player is on the path to ruining the game for everyone else. The words “Chaotic Stupid” do come into the alignment debate from time to time, and there is no real excuse for a player to go down that road!