I understand what you are saying, he doesn't show any real signs of ruthlessness that we see on the screen.

But he was there when they killed the hooker in his brothel, correct? Also, he did knowingly hurt Michael (his family) on some level at the very least, when he went behind his back and made some sort of deal with Roth/Ola?

I can't say I'm totally convinced he is not capable because it's not in his character (although I get the point).

If I had a reasonable explanation as to what Fredo thought he was doing when there was "something in it for me", or a reasonable explanation as to who else could have possible opened the drapes, I may buy that Fredo didn't know. But again, I don't have these explanations, and I do believe he showed the resentment toward Mike. Therefore I can only draw one conclusion.