I think that's a really, really bad idea if they're on Russian territory. Would create a precedent for the Russians (and other adversaries) to do the same to US citizens.
I agree that it probably would be a bad idea. That said, Russia executes state enemies on foreign territories already. And Vietnam kidnapped an asylum-seeker in Berlin in 2018. Another example, are the recent kidnapping allegations against Iran (https://www.justice.gov/usao-sdny/pr/manhattan-us-attorney-a...).
