It's been a while since this topic came up. But my short answer is: It's usually a combination of business and personal. Michael assured Sonny, Tom and the caporegimes that his plan to kill Sollozzo was "business, not personal," and he made a logical case why he--technically a "civilian"--would have a good chance for success. But he also seethed with personal motivation to protect his father and to avenge himself on McCluskey for breaking his jaw (the novel describes a "delicious icy chill" that went through Michael). You might say almost the same about the killings of Carlo and Fredo. The "business" reason was never to give a traitor a pass. Hagen explains this to Kay very well near the end of the novel, but the short explanation is that gratitude is the shortest-lived of human emotions, and that a forgiven traitor will be a danger to the family for as long as he's alive. But it's impossible to separate the personal--Michael's need for revenge--from both killings.