Personally, I think the crime in question should be the threshhold for how far things are pushed. Illegal camping resulting in a man being shot to death is an escalation that should have never been. That's my opinion. You are certainly welcome to your own. He was a man with mental issues. Perhaps a taser could have been deployed and everyone would have went home/to jail that night.
The reasonableness standard provides just what you speak of. It compares the governments interest in making a seizure versus the type and severity of force used. Had the officers just walked up to him and shot him dead that would clearly be considered unreasonable. The man pulled knives and made threats toward the officers which escalates the situation and has a dramatic impact in the above mentioned comparison. I'm not saying I agree with the way this went down, I see some very real issues personally. That does not change the fact that the man could have been cooperative and expecting the police to simply go away when someone decides they don't want to be bothered is not reasonable either.