To be honest, I'm not entirely convinced Michael would have had a happier life had he ended up in the "legitimate" world. There are plenty of law abiding people who end up with more regrets than he did. I think his tragedy was not simply a matter of going into crime, it was caused by other decisions he made as well. He didn't have to marry Kay, for instance. He should have realized she wouldn't be a good match for him if he was going to stay in the Mafia. It's interesting that Henry Hill found a wife who, while also not born into the wiseguy world, was much more compatible with it than Kay was (Henry and Karen were together for 24 years, impressive by any standard). Michael also didn't have to refuse to let Kay have the children when they divorced. Couples get divorced all the time, but it's not always with the bitterness that existed when Michael and Kay split up.