I know others may feel differently, but ME2 removed most of the black and white and filled it with grey. I liked most of the characters by the end of the game. You can justify the "goodness" or provide reasons for the actions they've performed, but the things that most have done are considered evil.
Some examples:
Morinth - kills people, but does it through addiction due to genetics. She had little choice for the genes she was given, but did have a choice not to become addict. However, addiction is a powerful force that is controlled biologically & subconsciously. Even if the conscious mind knows accepts there is no rational justification for it, the urge is hard to resist. Does this make her evil? Well, killing people without law on your side puts in prison IRL.
Jack - kills, steals, kidnaps all because she had a harsh childhood. Yes, she had a hard childhood and when you strip away the front she's human afterall, but other people have hard childhoods and don't end up that way. Again, these crimes put people in prison.
Thane - an assassin. As nice as he is, as much as he wants to repent, he killed people. Now, some of that was sanctioned by the Hanaar (so potentially law on his side), but some weren't. What gives him the right to decide if someone lives or dies?
Samara - kills for a code of justice, but is willing to kill those who store her from seeking justice (such as police with the law on their side). Killing an innocent person who will not let you kill someone is still killing an innocent person.
Garrus - for the most part is a good person. However, on two occasions he's either killed someone rather than bring them to justice or had to be talked down. That's either murder or attempted murder.
Personally, no matter how nice or friendly a person is, what their past is, what disease they suffer from, IRL an evil act is an evil act.
So, who do I think is the most evil - TIM. Who knows how many murders he's orchestrated, what crimes against humanity he's committed or how many lives he'd be willing to sacrifice simply to better himself... I mean... humanity.