Steelcan wrote...
Jack can join the list. Wrex was a mercenary for centuries, Miranda was the second in command of a notorious terrorist organization, Morinth makes Jack look cuddly by comparison, Samara kills without remorse or hesitation, Zaeed....well he's Zaeed. Jack is pretty par for the course
And
Thane, a Drell master assassin who's been doing his job since he was 6 years old,
Garrus, an ex-C-sec cop who went vigilante on Omega and carried out extra-lawful killings (doesn't matter what his intent was, it was still outside legal bounds and thus murder. Even if Omega has no government),
Grunt, a cheery baby-adult Krogan that likes to smash things,
Mordin, a former STG Operative and Renaissance-man-esque scientist responsible for reintroducing the Genophage.
Liara becomes the SB in ME2, and prior to ME3, she is pretty ruthless.
Kasumi has probably had to kill in her line of work before too. And since she's a thief, it is murder and robbery.
Jacob also worked for Cerberus (though I doubt he took part in anything too extreme).
James even killed an entire colony (indirectly) to get information about the Collectors. Who knows what
Javik has done...
The only "strait" characters so to speak are Ashley, Kaidan, and Tali. And they're some of the worst characters IMO.
I didn't include either Legion or EDI.