I think that the movie doesn't go into enough detail on Vito's rise to give us the depth of his ruthlessness. While it's said in the novel that Sonny made up for what Vito lacked in that area, I also remember reading that Tom was surprised to find out that Johnny Fontaine was screwed by some records piracy that Vito was involved in. He had no problem doing that to his godson, so what else was he capable of??