FAQ for
Firewall (2006) More at IMDbPro »

Cox wanted to make Jack and his family think that after he got his money, he would let them all live. From observing them at the start of the film, he knew they considered Rusty to be a part of their family. If he had killed Rusty, they would've know that he had no intention of letting them all live. Thus, his plan would be ruined.

This is one of the problems Richard Roeper had with the film.

Both Firewall and the original title The Wrong Element refer to the same thing. Cox and his men who stand between Jack and his family.

Willy was shot by Cox, Liam was beaten with a blender by Jack, Harry was shot by Cox, Vel was shot by Cox, Pim was hit with Janet's car by Jack right before the car explodes, and Cox was stabbed in the back with a pickax by Jack.