Actually, the question of whether Sonny would've killed Fredo is sort of a useless one...because the guess of about 100% of the respondents would be no, of course he wouldn't.

But until we saw that Michael had Fredo killed, none of us ever in our wildest imaginations thought he would actually do it.

So sure, we're all going to give poor, departed Santino the benefit of the doubt and say no, of course not he'd NEVER do such a thing.

Because he didn't live long enough for Fredo to have given him a reason to do it.

Apple