I thought this would be an interesting question to ask.
which one is the least evil? I do honestly think Deidara is the least evil of them. There are a few reasons why I think that.
For one, we know he didn't want to join Akatsuki, and was forced to join. I mean he was still a bomber before that but probably on a lesser scale, he certainly wasn't part of some huge thing, and he hadn't been disguarded by his village(his headband didn't have a slash on it). So they hadn't kicked him out.
Also, to me it appears he doesn't like killing people for the fact of killing really. He just loves his art.