Unfortunately, it is extremely common for DMs to do precisely that and fall a paladin because they slew a man who was unarmed, surrendered, or a prisoner, regardless of the fact that they are evil and would have been executed for their crimes anyway.
I actually played with this in my game recently by having an evil outsider try to get into the Paladin's headspace by intentionally going fully helpless after he was tripped during a duel with him by another player and going on about letting the hate flow through him. Trying to goad him into rage and being a prick.
The paladin proceeded to beat the fucker to death with his fists, and at no point was rebuked for this. This was because the thing he beat to death was an unholy vile piece of shit, no matter how it tried to play mind games and moral relativity. Hell, he was rewarded for this beatdown since the gang gained a level.
Good does not mean you have to be nice. A paladin's oath is key, yes. They often do serve gods that show mercy and kindness yes. But they are their deity's sword; in most cases they are not in conflict with oaths if they ice fuckers. It's when they break tenets or are knowingly doing shit I usually have the god look at them and go "the fuck man".
Kinda surprised that so many GMs would go after Paladins; seriously, what's with the Paladin hate? I never got it myself...
Also, now I'm curious; are there any other classes that get bashed by killer GMs? I know Cleric's got some hate from Reddit Atheists; any others?
It's because of two factors.
The first is that moral code and oath are easy to shift levers for the DM to play with to make a compelling arc. A paladin sometimes faces a no-win scenario where their drive to do right is in conflict with their drive to follow their vows or the law of their land. This in the hands of a competent DM can be fun.
The problem is the DM though. A lot of DMs hate paladins due to seeing them as "buzzkills". In my experience this sort of shitty DM is usually a chaotic stupid player usually outside of the screen, or has some idiotic hangups with religion in general. They might be smug relatavists who forget objective evil and good are in the setting for example.
Another is the DM who just likes to be a prick and make them suffer unfairly since they want to flex. This type is just poison no matter what you play; since they're just as likely to fuck over a rogue for trying to scout, or ding a wizard on books or just shove stuff that counters them.