No, Kasztner was not a tzaddik. And judging people who lived in those times, under those conditions, is, in my opinion, somewhat beyond our understanding. In any event, we have a Dayan Emet.
Many people made very bad decisions in those times. Kasztner has been singled out for vilification, especially by the Hareidi antiZionist community, who have consistently ignored the mistakes that their own leadership made during those times. Hypocritical, to say the least. Especially since they claim such wisdom (approaching infallibility) for those leaders that they may not be criticized. If they possessed such wisdom, why were their decisions not better?
This does not denigrate the wisdom that the Hareidi Rabbinical leaders do possess. But they were wrong on this occasion. People who believe that all Jews should be held accountable for all actions during the Shoah must include the Rabbinical leaders. Those of us who believe that we should not judge our fellow Jews for their actions and instead place all blame where it belongs–should include Kasztner in that accounting.