Black Lives Matter is in response to the massacre of black people by white cops.

Anything other than supporting it is racist bullshit. There's no reason someone would have to justify saying Black Lives Matter, unless you're a racist who doesn't think black lives matter and need to interrupt people when they're talking about it, further derailing any discussion of racism.
Blacks are not the only group of people shot dead by the cops in the United States. A lot of Non-Hispanic Whites, Asians, Hispanics and Native Americans were also shot dead by the American cops in the year 2015.
You need to see this:
http://www.theguardian.com/us-news/ng-interactive/2015/jun/01/the-counted-police-killings-us-database#A total of 1,140 people were shot dead by the cops in 2015. Of them,
578 (or more than half of the total) were non-Hispanic white.