You need to watch it again.
Michael has divested all corrupt activities, gone legit, paid everyone their share so that he can ease his conscious with being a parent of sorts and move on, especially onto the oil thingie with the church.
But the evil forces can't seem to prosper without Michael and move against him, so...
He moves against them, and sure enough he get's 'em - but at a price.
http://cdn0.hark.com/images/000/383/152/383152/original.jpg
The purpose is to show the audience that in the world of corruption via orgainized crime there's no getting out once you're in.
So don't get in.
It's a morality tale, not a cool subculture tale.
Same for REQUIEM FOR A DREAM or TRAINSPOTTING or MIDNIGHT COWBOY.
The lifestyle looks like fun, but it ain't.