Interaction with police is one of the rare cases where self defence doesn't cut it. In fact, defending yourself just gives more justification for law enforcement to apply more violence.
Imagine if you were George Floyd, and you knew you were going to get killed. What do you do? If you don't resist you die. If you do resist you might still die or at least be assaulted, then charged with resisting arrest and will have "deserved" the violence by resisting at the end of it.
On the bystander side too. Everyone watching knew it was wrong, but there is no good way to intervene. You either put yourself in harms way, and probably won't effect the outcome, or intervening action will be used to justify whatever violence was used, or likely both.