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Well, any pokemon following the commands of an evil trainer can be evil. It's clear they can choose not to follow a trainer's orders (like if your pokemon is above level 30 but you don't got enough badges and shit). By receiving an order and following through with it, you are complying with evil. Same reason why Peter Von Hagenbach (knight who was ordered to betray his king by the Duke of Burgundy) is evil. He was ordered to be a traitor and instead of going "Fuck you, I aint doing that shit" he went along with it.
Some pokemon can be evil without outside guidance form trainers, though, like that one Malamar.