Khrushchev, Beria, Yezhov, etc. It really feels like they didn’t do much quality control, if at all. I know Stalin was, contrary to popular belief, quite trusting. Especially in regards to Bukharin and the likes. But it’s not like he had the power to really kill anyone he wanted, again contrary to popular belief, even if he wanted to (at least any politburo members), so his personal views of his colleagues don’t matter.