We all know that Michael was totally against the Family organisation. But after all he voluntary drawned in the business and became a good Don.
But why do you think, he changed and became someone he didn't want to be?
From love to his father and family, or felt he just accountable because no one except him, had the skills to replace the Don.

He took the attack on his father very personal, and he probably felt he was best suited for protecting him and getting revenge.
If you remember outside the hospital, he also had nerves of steel. This was probably from his war experience.

I agree......I think Michael knew that his dad's time was nearing and end, and someone "smaht" would have to take over if the family was to survive.....Sonny could not have done it alone.......Michael was confident he could make it happen.

Who says Michael "changed"? His father's shooting impelled him toward killing Sollozzo and McCluskey. But that move, and every one he made after that, was his own choice. At every turn in his life, Michael could have chosen other paths, made other decisions. But he chose the life he led. As the Don said (in the novel): "A man has but one destiny." Michael's was to be a criminal.