There are a lot of people who still believe that Mike Brown was murdered. The report by the Obama DOJ doesn’t mean they’re wrong; like the Trayvon Martin case, none of us who were not there actually KNOW what happened.
If Warren believes that Brown was murdered, well, that’s her opinion and it has as much validity as anyone who disagrees with her.
Personally I have no idea what happened to Mike Brown though I had questions I raised at the time. But because police in this country, and especially police in Ferguson, have a terrible reputation of institutionalized racism against young black men including numerous murders in the past, it doesn’t surprise me that any reasonable person would simply begin by assuming that this was just another example in that trend.