I'd like to bring up a point- corrupt priests. In a series of fantasy novels I've read, the protagonists belong to one of several knightly orders devoted to a large, organized church. They soon realize they are dealing with a conspiracy involving a corrupt church leader who has their eye on the papacy- the current pope is old and sickly, and the next one is decided by vote.
Using bribes and blackmail, the evil priest has bought many votes, and is using their influence to cause a great deal of problems for the heroes.
Now imagine a scenario like this in a D&D setting. If the instant a priest steps out of line they lose their powers and cease to be a priest, this becomes immediately problematic- unless they are allowed to choose a new master, of course (the local dark power will probably be recruiting, be they Asmodeus or even that schmuck Cyric).
Another point to consider is that most fantasy deities aren't very well mapped out. We mostly only know general things about them. There is no printed Bible of Tyr, Pholtus, or Dumathoin laying around, after all. Expecting a player to devote themselves fanatically and faithfully to a made-up imaginary deity who has, at best, a wiki article somewhere and nothing like actual commandments, holy days, ceremonies, tales of saints, and the like seems a bit much. Now if you, as a DM, can offer all of this to a player, that's one thing. But if someone says "Man, I want to be a Death Domain Cleric" and all they have is a list of deities to choose from who have that Domain, well, there's probably a big difference between serving Kelemvor, Wee Jas, The Raven Queen, or Nerull!
It's up to the DM to describe, when the player makes their character, what they expect from the Cleric, I'd think, and if the player seems like they are chafing under the mandates of their God, you should suggest a different path.
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A classic plot point in the Hellraiser series is that John Constantine (a successful Warlock if there ever was one) has made deals with multiple powerful demons, which leads to his masterstroke when he finally dies, and it causes a civil war in Hell as each demon tries to collect what they are due! A Warlock absolutely can work against the best interests of their Patron- wriggling out of a deal with the Devil is a classic literary trope, after all. And why couldn't a Warlock make more than one Pact anyways?
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The Paladin breaking their oath, and therefore losing their exalted status is trickier- the myths and legends that inspired the D&D Paladin are rife with cautionary tales about falling due to pride, temptation, or hubris. Going on epic quests to atone for such transgressions is part and parcel of the fantasy of a holy knight. This need not be very strict- minor transgressions can be easily forgiven, so long as the Paladin continues to strive towards their chosen ideal- mortals are fallible, and no one should be demanding perfection from them.
That having been said, it occurs to me that any Paladin who is likely to commit major transgressions of their Oath should never have gotten powers in the first place, because obviously they don't believe in it very much! It seems to me that if you want the player to hew closely to their Oath, this all should have started when they joined the campaign- you should ask them to consider their Oath carefully, and explain what it means to them. What circumstances would lead them to betray their Oath?
If they are incompatible with the Oath, you can simply tell them that, no matter how cool the powers are, this might not be the best thing for them to roleplay.
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Being advised not to strip player characters of their abilities, and/or having little guidance for what happens if they do things that seem out of character is not some great sin against the DM- if you want the actions of the PC's to make sense in your game world, then you are free to set expectations. But there's more than one possible outcome here, it need not be "ah! You didn't accept that goblin's surrender, you're not a Paladin anymore!". There are other ways to handle this that aren't so grossly punitive. You shouldn't even need to wave such a blunt instrument around, to be honest- if someone is a player worth having in your game, you should be able to have a conversation with them about expectations and consequences.
If they're not, then stealing away their powers or striking them down with Gygaxian "bolts from the blue" will likely fail to make them a better player, and you'd be better off just asking them to leave the game.