I realize the locals are not compelled to help but why would they release such a dangerous person, one who has no right to be in the USA in the first place, back into the community when ICE is perfectly willing to take them off your hands and out of the community.

I imagine because they consider the impacts of cooperating with ICE as not being especially severable, and that they outweigh the potential harms in those cases.

For example, one negative effect of having your state and local law enforcement cooperate with ICE is to reduce the degree that folks from the undocumented communities - and their family members and neighbors - will cooperate with your state and local law enforcement. If those folks stop trusting the police, it can hurt law enforcement efforts within that community. Local elected officials may determine that negative impact will be worse than the possible negative impact associated with not cooperating with ICE.

I don't know whether the answer is the right answer. I expect it may vary from community to community, and it might vary depending on the relative proportion of "worst of the worst" vs. ordinary folks that would be eligible for detainer and the resources necessary to try to figure out where to draw lines. But as with most things, if you look only at the very most extreme examples within the subject population (like a "multi pedophile") when a policy will have impacts on a much larger population, you'll get the wrong answer.