He was not given "sanctuary" and he most certainly wasn't given a Carte Blanche to murder. First and foremost, there is almost no indication that he murdered anyone. That would imply that he had some level of intent or forethought. Accidental death would be the only thing that would come to close to describing what happened. Second, he was convicted of a crime, so he was also not exempt from the law.
Also, the whole aspect of Sanctuary City, is a tool to improve Law Enforcement, not to diminish it. Local law enforcement are not there to police federal matters. They have enough on their plate already and there is a reason those areas are separate. If the locals living in an area are not willing to trust and cooperate with police for fear of deportation then that only makes their jobs harder. I don't want local police involved in federal matters any more than I want National Guard planning for a military invasion. It's not their responsibility.
This case should have never achieved the national stage it did. There was never enough evidence to support a murder, and the evidence never pointed in that direction in the first place. Instead of being about illegal immigration it could just as easily have been about gun control, the incompetence of the DA's office, how the police handled someone who was obviously mentally ill, the proper handling of firearms by federal agents or any number of other issues that came up. The very title of the article that is posted at the start of this thread easily shows an agenda as opposed to an effort to seek the truth and find justice for the victim.