Here's what sanctuary city status means, and while bellevue isn't one I believe it wishes to follow a similar principle:

When someone is a victim of crime or a witness to a crime they can speak to the local police without being questioned about their immigration status.

This benefits the community as a whole since more people feel free to speak to the police. Imagine you were a victim of a violent assault and a witness is undocumented, they should be free to give evidence with fear of being deported. It does not protect people who commit crimes in anyway.

In the sad case of Kathryn Steinle's murder this was a failure of the Federal Government since ICE had clear opportunity to deport the perpetrator rather than return him to San Francisco where he killed Steinle.