Right, literally no other way this could have been handled. FFS, in the moment just before he's shot, there was plenty of time to at least try tasing, pepper spray, or other means on him first. Things were not out of control at that moment in time; the cop had successfully pushed him back and got some space between them with just a baton, his partner came to back him up, and the guy with a chain was standing still several feet away. There's no way the situation would have unfolded like you describe if there was another cop on the scene, which there was. Shit like this is justifiably why people don't like cops, they always resort to lethal force whenever they feel even slightly threatened. We saw for ourselves that a simple baton was proving pretty effective. With another cop, getting control of the guy to make an arrest shouldn't have been very difficult.

You said yourself that the chain itself wasn't the real threat here, it's that the victim might have tried to go for the cop's gun, but that applies in literally every situation. You aren't describing the danger of encountering someone with a chain, you're describing the dangerous nature of police work. It's exactly what they knowingly signed on for. Even when the threat is not readily lethal and other methods would likely be just as effective, plus not result in anyone's death, cops still reach for their gun without a second thought. You'd think supposedly trained professionals would have a little bit more finesse and expertise. Why should we let cops be judge, jury, and executioner for crimes that wouldn't normally carry a death sentence if they actually did their jobs right?