They were almost certainly guilty of being too violent. Violence per se, when arresting someone who is not coming willingly, should be considered acceptable in most cases, but in this case it was too much.
But to label the officers as murderers is a gross overstatement. They had no way of knowing that this tactic, the chokehold, would kill. For the vast majority of people, a chokehold is not life threatening. He may have been obese, but they had no way of knowing that he had asthma, sleep apnea, etc, that he was fragile in any way. They may have been guilty of manslaughter, but anything more is just being ridiculous.
And to tar all police with one brush is similarly stupid. Not all officers are the same, and most are responsible and hardworking. The officers involved in this case were definitely stupid, following what they probably thought was procedure, were overzealous, and at that moment they suffered a common sense failure. But to say they wouldn’t have done the same in an identical scenario in which the participants were all white is way too far.