Yeah, i can understand dumping 10+ rounds into a dead body on the floor. 
I don't KNOW why he did... but I would like to participate in the game.
A few possibilities
1. Since only about 25% of cops ever fire their guns in the line of duty, this was overwhelming his neurological abilities and he fired until he was empty because well, damn.. some guy with a gun is in the station...
2. He considered that he was in an office with a separation between himself and the bad guy with the gun so just running up with the bad guy at gun point and kicking away the gun and putting him in cuffs to end the threat was not an option, he decided that insuring death was the only option.
I got no issue with either reason.