A really weak argument that is purported by every bigot trying to avoid the issue, and that's exactly what it is, a red herring. A tax-funded system that is disproportionately targeting, killing and incarcerating people of a particular skin colour is different to civilians murdering other civilians, for one. The former is not related to the latter, nor does one invalidate the other. They are both issues. Before I go into the reasoning why tax-funded, predominantly white cops killing disproportionately killing black people is a problem, I just want to point out that white on white crime is also a social phenomenon, according to your argument.
The primary issue #blacklivesmatter has with 'police brutality' is that there is a bias in policing black people compared to white people. It makes matters worse when you realise the obvious; working black communities are essentially funding a system that is targeting them. You may not agree, but the facts support my argument. According to the Department of Justice report (2001), from 1976 to 1998, black people were 5 times more likely to be shot by a police officer than white people. Combined data from Substance Abuse and Mental Health administration and a 2009 Human Rights Watch report found that despite white people using just as much drugs as black people, black people are incarcerated and given harsher sentences for drug possession. There's a really interesting report from the Bureau of Justice Statistics in 2007 which also outlines much of the statistics around the overrepresentation of black people in prison, the harsher sentencing and the higher likelihood of being arrested and shot by the police. It becomes hard to dispute the facts when you look at the surmounting evidence.
But the evidence isn't the problem, as seen in this thread, it's the excuses and *****y explanations used to justify these statistics. Much of the arguments against #blacklivesmatter are similar to the OP, and mostly root themselves in the idea that black communities are inherently more violent than white communities, which somehow warrants them being shot more? In actuality, the evidence from Parker (1989) and Smith (1992) suggests that African Americans don't endorse violence any more than any other community. Furthermore, Hannon (2004) reviewed approximately 900 cases and founded that black perpetrators were no more likely to respond with violence in the face of minor transgressions. The argument that cops should feel more on edge when dealing with black people is completely unfounded in the research, and is, in my opinion due to sub-conscious racial bias. Interestingly, Correll, Park, Judd and Wittenbrink (2002) simulated a violent police-citizen interaction, and asked participants to react by either not (fake) shooting or shooting violent and non-violent, black and white 'citizens'. The participants were much more likely and faster at shooting the black, violent 'citizen' than the white, and took longer to decide on not shooting an unarmed, black 'citizen', than an unarmed white 'citizen'. Point is, blacks are NOT predisposed to being more violent than any other race or ethnicity, but they're being dealt with by police officers as if this is the case. The entire legal system reflects a racial bias where black people are being incarcerated and killed more frequently than their white counterparts, and little is being done about it.