The first time Tommy killed somebody was to save Pat's life. While he is seen as a loudmouth by most, Tommy knows it is to cover up his insecurity. He is mostly accepted by Hacken and Ringo Chen because he is Sean's nephew and Tommy's protege.
With the coming of Natt the Hat, Pat began to feel that his friendship with Tommy was crumbling. He therefore decided to be tougher to show Tommy that he was a worthy friend. He unfortunately got to prove it when he was tortured and ultimately killed by Johnny Navarone, a hitman sent by Moe Dubelz to find and kill Tommy. Pat never gave in however, thereby showing how tough he was and that he was a true friend to Tommy to the very end.