Although Sonny was famous for his bad temper, he could be a soft touch. He was the type that reacted, and then often felt bad for his reaction (witness how bad he felt after insulting Tom. Michael didn't care when he did so). And, as DC said, Vito would prefer to reason, and only used violence as a last resort. Michael, on the other hand, was far more ruthless. He waited patiently to regain his own brother's trust, and then had him murdered. If he could wait so patiently to take vengeance against his own brother, then killing a stranger would bother him no more than swatting a fly.