Thankfully there are already laws to protect people being persecuted, in danger, people needing asylum, etc... We need even better laws in these areas and improvements in witness protection laws for a part of your example. But again sanctuary cities "work" for now but it is not a long-term solution. Beyond attracting criminals, it also creates a weird lawful oxymoron at the opposite of the rule of the law. (And again, there are things like asylum, etc...)