Uh, it's a little hard for me to figure out exactly what you mean from the above, scarfacetm...BUT:
Roth would have been interested in Fredo not because he looks "innocent" or "sweet" but because he was close to Michael and had info Roth valued. Roth would know that Michael would figure out, sooner rather than later, that Fredo was the traitor, but by then it wouldn't matter. In fact, if the machine gun attack on Tahoe had succeeded, it would have been all over. At that point, Roth would have won, and it wouldn't have mattered to him whether Fredo's role was found out or not.
Fredo's "sweet" and "innocent" appearance is meant to appeal to us, the audience. It enhances the drama to find out (to our enduring horror, as attest all the posts on this subject), that Michael's very own brother was a traitor. It makes his betrayal so much more significant to us than, say, Carlo's or Paulie's.