Too true, TB. In the novel, the Don tells Michael that men like Luca and Al Neri are violent, dangerous men. They don't have any fear, not even of death. Their violent tempers actually have them asking for death each and every day. Since they fear nothing, not even death, the only fear they have is that their Don will be the one to kill them.

The Don always made sure to treat Luca with respect and dignity, but I think that, on some level, Luca's violent nature disgusted the Don. He preferred not to think about that side of the business, but I believe he felt that Luca was necessary to carry out certain things. The Don didn't like violent anger. He preferred subtlety.