Maybe Garrus has a better undestanding on Drell culture and all it implies for an assassin. I don't think Garrus holds murders for money in high steem, but Thane's situation is quite complicated
(the writers made sure we got that). So, Garrus probably respects Thane, because he knows or at least has heard of the Hanar using the Drell. Also, I don't think he disapproves of Thane killing the ones that murdered his wife, for obvious reasons.
It's likely that Thane admires Garrus for his job on Omega, and his wish to do good. I wish they interacted more, because right now, I feel their relation is very... Cold. Almost unexistant. I notice that whenever I try to pair two characters and it doesn't work very well... I'm just saying that somebody should try it...