The novel also tells us He really had to go, his bowels were loose -- not so icy! now

Originally Posted by Evita
Extract: True Sonny and Tom didn't ask Michael to kill Sol and Mac-- Michael volunteered to kill them--he wanted to kill them, to save his father's life

I too reckon, there was no other [viable] scenario” they can't wait, because Sollozzo was going to kill Vito ("That's the key for him.")
My take too, Sonny and Tom would have left "Mike out of this..." if there was any "other [viable] scenario”

Business or personal?
Originally Posted by The Last Woltz
Extract: He was angry and terrified that his father might be murdered. He wanted to get McCluskey back for breaking his jaw. So, in that sense it was personal. But it was also the best move strategically. They did need to kill the Turk and there seemed to be no other way than for Michael to do it himself.