And if I recall, Vito tells Michael that men like Luca don't fear death, in fact they beg to be killed. However, if you can become the person that they fear will kill them, then you will have their loyalty forever. That is how Michael gains the trust of Al Neri, who someone (Tom perhaps?) refers to as Michael's Luca. I believe that is why Luca is such a complex man and an interesting character. How many of us don't fear death?