Did he or did he not have a gun? If he did have one and that's what they grabbed from his pocket/side, it's such a tough call.
People forget that these police officers are regular people like you and I. They go home at the end of the day to their wives and kids. When you respond to a call and for whatever reason the altercation gets physical and a gun now because involved, I can't even imagine the thoughts that must be running through their minds. They want to make it home safely to their families, and perhaps sometimes they make the wrong decisions in that unfortunate spilt decision.
Now in this particular situation, based off of this video a fatal kill seemed completely uncalled for. They had him on the ground, two against one. Even if they felt they couldn't control him, I think a hit with a taser would have subdued him. Again though, there could be a million different reason why this all happened and played out the way it did.
I'm not sticking up for the police, at the end of the day a man lost his life and it's very sad but I can understand a cops mindset when a weapon becomes involved during an altercation.