Like basically all Pokemon, it depends. They're not evil unless people are. Like Team Rocket's Pokemon said years ago.
That sure explains Mystery Dungeon and the random evil wild Pokemon in the anime
Pokemon's worldbuilding and philosophy has always been lackluster in general, and the Pokemon themselves float variously appear as people, pets, and livestock. The early anime went out of its way to sell the Pokemon as non-hostile, which resulted in things like Team Rocket's Ekans declaring that Pokemon were only evil because they served evil trainers. Pretty much every movie and episode have gone out of their way to showcase hostile Pokemon as being misunderstood-at-worst by introducing complications and misunderstandings, which has perpetuated the increasingly babyish notion of No Bad Pokemon ever. Off the top of my head I can only think of two (2) anime Pokemon who were actually villains, the Teddiursa/Ursaring from a Johto episode that manipulated people for the purposes of thievery, and a Sinnoh-era Togepi that was so cartoonishly evil it stole a f*cking rocket-ship.
As far as the anime's concerned, the only evil Pokemon are a thief and a joke.
Even the main games have gotten involved in some whitewashing. The big example is Giratina, who is basically the Satan archetype in Pokemon form, but Darkrai got some of it too when
Platinum went out of its way to sand off their edges, having Giratina interfere with Cyrus and explicitly declaring that Darkrai means no harm. Which in a rather Lovecraftian irony dooms game-Darkrai to a lifetime of misery and guilt due to its own innately horrific existence (Darkrai makes for a good study in the pitfalls of shallow niceness).
In
Mystery Dungeon, on the other hand, it's all Pokemon, all the time, so it needs the Pokemon to be people in order to even
have drama at its preferred level of complexity, and is refreshingly honest about its villains, which is a plus.
