It was clearly an extra-judicial execution, just like Christopher Dorner. They cornered someone they really wanted dead and ignored every convention and law to kill them. In the case of Dorner their thirst for vengeance led them to shoot and murder multiple unrelated people. [1]

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...