I believe Michael was against the use, possession and trade of drugs. He understood the amount of distrust and backstabbing within the world of drugs. He also disliked the race-mixing (similar to Joe Bonanno) because it broke down the tradition and the rules of the secret society.

If his plan was to stop the drug trade he would ultimately fail.

But this is The Godfather , he could of opened a fish shop the next day if Coppola wanted.