Nothing bad could ever happen from cities and states having "sanctuary" policies, right?

Yeah, or maybe not. Letting people go from police custody just because they are illegal immigrants ignores the fact that maybe they should be held anyway, regardless of their immigration status. Like if they're members of a violent gang and have been arrested before:

Last week, police arrested two teenagers, Josue Fuentes-Ponce and Joel Escobar, and charged them with the murder of 14 year-old Ariana Funes-Diaz. She was killed in a tunnel, beaten with a baseball bat and slashed with a machete. She was found naked. Reportedly, the killers, members of MS-13, ordered her to strip before they murdered her.
According to authorities, Fuentes-Ponce and Escobar killed the girl because they were worried she would report them to police. She had lured a man to Northwest Washington, D.C., where he was beaten, robbed, and interrogated about his gang affiliations.
This was not the first involvement in a robbery by Fuentes-Ponce and Escobar. Last year, both were arrested in connection with a robbery case.
Because Fuentes-Ponce and Escobar are in the U.S. illegally, ICE asked the Prince George’s County, Maryland Department of Corrections to detain them. It did not. Thus, these two MS-13 thugs were free to commit more robberies and to butcher a 14 year-old accomplice.

But hey, no problem with refusing to cooperate with immigration authorities, right?