This all started with Obama praising the violent criminal Trayvon Martin and supporting this behavior.
That certainly didn't help, but violent black people targeting police officers has been going on for longer than that. In 2009 we had four officers gunned down in Lakewood, WA by a violent black felon.
“The gunman was identified as 37-year-old Maurice Clemmons, originally from Marianna, Arkansas. Clemmons had a violent criminal history, with at least five felony convictions in Arkansas and eight felony charges in Washington. In 2000, Clemmons had his 95-year sentence for aggravated robbery commuted by then-Governor Mike Huckabee and moved to Western Washington in 2004.[12][13] In spring 2009, Clemmons was charged with rape of a child and third-degree assault on a police officer for punching a Pierce County sheriff's deputy in the face during a confrontation,[14][15] and was let out of jail on those charges after posting a $150,000 bail bond one week prior to the shootings.”
https://en.wikipedia.org/wiki/Lakewood,_Washington_police_officer_shooting
A female police officer had been murdered in Seattle a month prior under similar circumstances. Two more officers were killed in rural Pierce County not long afterwards. But certainly Obama has made things much worse. Unlike the psycho killer of the TV reporter and cameraman most of these executions were committed by people who obtained and possessed their firearms illegally. So more laws wouldn't have made much of a difference.