I'd say vehicular homicide is "serious", but sanctuary laws (and evidently mainstream liberal thought) disagrees. Convenient, that.

No, they don't. Sanctuary laws and mainstream liberal thought don't think vehicular homicide is not serious. Sanctuary proponents believe that many other crimes are not serious enough to warrant being thrown to ICE, especially the minor misdemeanor and drug possession charges that make up the majority of arrests.

So they've committed crimes on top of being an illegal alien. And by refusing the retainers, the local jurisdictions appoint themselves arbiters of who gets to stay and who needs to leave the country.

That's power and moral authority that sanctuary cities lack. Bigly.

They don't lack it at all. They have power over their own resources. They can't prevent the federal government from making immigration law or the federal government from enforcing it however they want. But they get to decide whether a particular use of their own resources is for the benefit of their community.

Which isn't the job of these local jurisdictions.

Steve has it right: if you don't like the immigration laws, change the immigration laws. You don't get to just ignore them because Reasons.

They're not ignoring immigration laws. There's no law that says that local governments have to comply with ICE detainers. ICE doesn't even claim that. These are requests, not statutory obligations. If ICE tried to turn them into orders, or Congress tried to enact a statute forcing states to do this, it would probably be unconstitutional. But we're not at that point. States are perfectly free to choose not to put their own resources in service of enforcing federal immigration law.