His death had nothing to do with the Constitution, it was because of a trigger-happy, confused cop who had his back to the guy, then noticed a gun next to him (that another officer previously put there). The US government had no part in that situation at all.
What are cops if they're not the government? Are they private corporations?
Of course cops are the government, but they're also humans that make mistakes and commit crimes themselves. But this isn't actually related to some greater constitutional crisis or anything, unless you have some evidence that suggests they were instructed to act the way they did... but I've never seen anyone come forward with that yet.
You have government officials tyranizing the population and now the argument is "of course it can't prevent that".
What good is it then?
The constitution is just some rules people wrote up generations ago. It's not a magic spell that has power by itself, it is only effective as a social contract as long as power structures agree to follow it.
If armed government enforcers break it with impunity, what good is it?