If you're going to use classic monsters in your games, the players will have certain expectations of how those monsters will behave, what strengths they will have, and what weaknesses can be exploited to defeat them. For some players, this is good and comforting. For others, this is uncreative and a path to boredom: "Ho hum, another kappa... We bow politely and deeply to it."

To mix things up and keep players on their toes, you need to adapt and modify. Make your evil faeries unaffected by iron, but vulnerable to copper. Have mummies wearing Rings of Fire Resistance. Take green slime and make it red so it's difficult to identify. This is all good fun and a great way to prevent player knowledge leaking into mysterious situations where their characters would be at a loss for what to try.

But there are certain lines that shouldn't be crossed.

Transcript

Padmé: PCs can't be evil!
Anakin: Anakin is somewhat mentally corrupted and preoccupied with his plans to the exclusion of others. I'm just playing—
Padmé: This is just like the monster game you ran! Only exactly the other way around!
Anakin: What?
Padmé: We thought they were evil monsters, only you wanted them to be sympathetic characters.
Anakin: Are we going to go through this again? I thought we agreed not to talk about that again.
Padmé: They were vampires! You can't make vampires good! They're evil, they have to be killed, that's all there is to it!
Anakin: Jim—
Padmé: And vampires do not sparkle in the daylight!!