How can a city say illegals will not be arrested nor will ICE be contacted why even have useless laws?

There is a growing movement in San Francisco to end it's status of Sancuary city. San Francisco has a habit of not turning in illegal immigrants commited of violent crimes to ICE. There was one murder and another violent beating on recent news in San Francisco from illegal immigrant with a felony. My opinion is that there should be a law that if an illegal is found guilty of a violent crime, they should be deported and if they ever show up in US soil, they get the death penalty. As for the two cases of violence in San Francisco, I hope San Francisco lose alot of money in law suits filed against them, and the family of the murder victim makes Mayor Newsom unelectable.