Look at the technology behind the reapers. It is at least millions upon millions of years more advanced than anything in the galaxy. You're not going to give the benefit of the doubt on the way things work on a galactic scale to those who've been around a lot longer than you could ever possibly fathom?.
Not when their evidence of that is shit like the Collectors, husks, banshees, etc. and my evidence is forging alliances that seemed impossible. I'm not going to give them the benefit of the doubt when they say they want to assimilate me in capsules that traumatize survivors and turn me into some kind of mush for their benefit. In fact, I am going to do the opposite of giving them the benefit of the doubt in that the cruel destruction of advance organic life forms benefits anyone but them and take my risks with synthetic life forms, which thank to the reaper influence on them (ironic) have become even more a helpful and understanding species.