Probably not justified. The reason is that such person would need to take out all the police present as a logical consequence. Or at least, that is what I think would likely happen. One must consider the downstream consequences and risks. Is one person's life worth all the killings that would happen? Possible scenario could be like this:
Person shoots Chauvin. Other officer shoots them. Officer misses and hits citizen. Other officer fires back and hits intended target. First guy shoots again and kills second officer. Officer shoots again and kill shot his first guy. It could also be worse. How many people could die? How many other innocents than Floyd?
It would probably make more sense in an isolated situation of just Chauvin, just Floyd, and just a citizen with a gun. If Chauvin in that case refused to stop killing Floyd after being given chances and proper information that he was killing him, then tackling Chauvin would be appropriate. If he still was a danger to life and limb, then shooting him would be appropriate in self-defense.
This is my opinion, anyway and I am trying to reason it out.
The only way to have prevented what happened was for the officers there to have acted properly and kept each other in line. Instead, they all failed and either they need better training or to be fired. People can't be shooting at the police. Police are supposed to be trained how to apprehend people, even people that aren't cooperative, without them becoming dead in the process.
People shouldn't generally shoot police. Of course, you want to prefer other police to act, but if they don't then the consequences of your inaction result in a life lost.
Their actions are illegal and immoral. You have to save people's lives and hope that when it comes to it, the court will defend your basic rights to save a life in self-defense. That doesn't mean it's generally okay to be a vigilante, but in the case of self-defense of a life in immediate danger of being murdered, I think exceptions ought to be made.
Look, I've been in the shit with a drill sergeant telling half the platoon to attack one of our soldiers in the platoon. It was up to the other drill sergeants to do something. They didn't. Anarchy resulted. I stepped up and physically stopped the attack. It seemed like 20 versus 5, but we won and the drill sergeant told everyone to stop. Do you think I was wrong to defend someone? I felt like his life was in danger from an attack of some 20 people against him ordered illegally and I had to act within a split second to do something. I don't think I was wrong.
