Friendly reminder: no rule in D&D mandates your character's alignment, and no class is restricted to certain alignments. You determine your character's moral compass.

I see discussions that refer to such rules, yet they don't exist in 5th edition D&D. #DnD

D&D has general rules and exceptions to those rules. For example, you choose whatever alignment you want for your character at creation (general rule). There are a few magic items and other transformative effects that might affect a character's alignment (exceptions). #DnD

Your character's alignment in D&D doesn't prescribe their behavior. Alignment describes inclinations. It's a roleplaying tool, like flaws, bonds, and ideals. If any of those tools don't serve your group's bliss, don't use them. The game's system doesn't rely on those tools. #DnD

Regarding a D&D monster's alignment, here's the general rule from the Monster Manual: "The alignment specified in a monster's stat block is the default. Feel free to depart from it and change a monster's alignment to suit the needs of your campaign."

TL;DR: The DM decides. #DnD

Want a benevolent green dragon in your D&D campaign or a sweet werewolf candlemaker? Do it.

The rule in the Monster Manual is that the DM determines a monster's alignment. The DM plays that monster. The DM decides who that monster is in play. #DnD

"What about the werewolf's curse of lycanthropy? It makes you evil like the werewolf." The DM determines the alignment of the werewolf. For example, the werewolf you face might be a sweetheart.

The alignment in a stat block is a suggestion to the DM, nothing more. #DnD

"What about the Oathbreaker? It says you have to be evil." The Oathbreaker is a paladin subclass (not a class) designed for NPCs. If your DM lets you use it, you're already being experimental, so if you want to play a kindhearted Oathbreaker, follow your bliss! #DnD

Even though the rules of 5th-edition D&D state that players and DMs determine alignment, the suggested alignments in our books have undeniably caused confusion. That's why future books will ditch such suggestions for player characters and reframe such things for the DM. #DnD

"What about demons, devils, and angels in D&D? Their alignments can't change." They can change. The default story makes the mythological assumptions we expect, but the Monster Manual tells the DM to change any monster's alignment without hesitation to serve the campaign. #DnD

"Why are player characters punished for changing their alignment?" There is no general system in 5th-edition D&D for changing your alignment and there are no punishments or rewards in the core rules for changing it. You can just change it.

Older editions had such rules. #DnD