People are responsible for their own actions. However, a man with a record of arrests, no indication of joining any protests, and may be mentally unstable, shoots his girlfriend in Baltimore and then travels to New York to shoot a pair of police officers, is not responsible. The blame lies with a political grandstander
Or: Leftists duck responsibility by blaming problems on larger social structures and externalities. Conservatives obviously don't do that when they, uh... try to portray the shooter as not actually responsible for his own actions, because he was just doing what other people told him to do. Even though no one, including Sharpton, actually told anyone to gun down police officers in broad daylight, or endorsed the use of violence against the NYPD.
Blaming this on Sharpton, or Di Blasio, or protestors is not about proper attribution of responsibility. The murder of officers Ramos and Liu is not a result of anything Sharpton, or Di Blasio, or any protests. If Sharpton did not exist, if Lhota or Quinn had been elected instead of Di Blasio, then citizens would still be livid about police abuse; there would still be national protests over the Brown and Garner grand jury debacles; and Brinsley almost certainly would have shot someone.
If you really want someone to "take responsibility," then perhaps you should support holding the handful of abusive NYPD officers accountable for their actions, and support revisions of police policies that make them less antagonistic to the communities they are supposed to protect.