He killed 5 cops and wounded 7 others at that point, maybe they were being slightly vengeful, but I also understand not wanting to send in more dudes.
In a democracy with rule of law, the police have to be held to a higher standard. If not, they are just an armed gang with a licence to kill. It also undermines any concept of negotiation or surrender. The next person will go out guns blazing.
[1] https://en.wikipedia.org/wiki/Christopher_Dorner_shootings_a...