A Pokémon is always born good and true. The only way it can turn is by having an evil trainer. Therefore, no Pokémon can really be classified as evil. Yes, I know that in various episodes, we've seen different Pokémon do bad stuff. For example, Mewtwo. It stole people's Pokémon and cloned them to have his own army. Does that not sound a bit twisted to you? Aye, but it wasn't doing it because it was bad at heart, but because it was mislead, particularly by Giovanni. A Pokémon can be easily influenced and can make mistakes, but that doesn't mean that it can be classified as evil.
However, looking at it from the point of someone who has no interest in or knowledge of Pokémon mythology, I wouldn't want to be left in a room with a Houndoom or a Murkrow.