That statement is true, but it weakens any argument that uses it.
Suppose you are pulled over for going 10 miles over the speed limit. And then the cop knocks you senseless because you hesitate in producing your driver's license, or whatever. The authorities can rightly claim that had you not been speeding, you wouldn't have been beaten. But that claim can not then excuse the the beating.
My hypothetical is certainly not close to the Garner case, but I hope folks get my point. The cop's defenders here need to use other arguments to make their case.
The main point is, with Trayvon, Brown and Garner, they all triggered the escalation of their situations. However, with Garner they did NOT resort to lethal force, Garner's own health problems are what killed him.