Vito didn't have the heart, and Sonny definitely didn't. He couldn't even kill his brother-in-law, who he hated. However, Michael could and did, and Vito sanctioned it. He knew he was leaving it in Michael's hands. So, while Vito couldn't bring himself to do it, he knew it was going to happen, and he probably was relieved to be passing that particular chore to Michael.

So, no, they couldn't. However, Vito also had the type of personality that encouraged loyalty. He would never have been the focus of such resentment because he had far superior people skills than Michael.