Which devil? I will assume you are referring to the Judeo/Christian devil, Satan, Lucifer or what have you. Though some may argue that Satan and Lucifer are two different entities. However, that is too much to go into. The question of is he evil and why it is thought so can be pretty simple. He is in rebellion against his lord. In the Christian Bible he is mentioned many times as the accuser, the deciever, leading mankind astray and so on. Yet, he has access to god where they have conversations about humans. One such in the book of Job he and the lord make a bet, yes, they are wagering on a human. Pretty cozy for enemies don't you think. He is also called the god of this world, if that is the case then any outside god would be the rebellious one I would think. Through centuries all manner of bad things have been placed at the foot of the devil, but strangely, it is usually those who claim the side of good who are doing the evil. I don't know, just rambling.
Yeah I mean Satan and lucifer because they both helped me in problems and even saved from someone. They aren't evil when I met them they were really good and treated me well. I didn't expected but they really helped me a lot.