It is reasonable to assume that the law is the problem.
There is no murder charge because the law lets police officers behave in ways which if done by anyone else would clearly bring a murder charge.
The US have a unique situation, where the assumption is made that the detained members of the public are assumed to be carry weapons, not assumed to be defenceless. So if you are drunk and do not follow orders the law allows you to be killed.
Clearly when the laws were made, it was not intended to result in all these innocent people losing their lives. The more video from Police and public documenting this waste of innocent lives will ssurely mean the rules of engagement will have to be looked at and changed.