He's not dead. Aside from that, the allegations are that that the guy had been randomly attacking people with hammers. And as we see here, he immediately attacked the officer, on sight.

Was it the best choice? No. But the whole thing happened in just a few seconds, start to finish. I can't fault them or say it was a bad choice - I would likely have done the same upon seeing someone I worked with being beaten about the head with a hammer.