Hurr durr bEcAuSe hE liKEs hiltlar
Killing Riddler would accomplish nothing except for paving the way for another smooth talker to take over Europe. If it wasn’t Riddler doing things at the time that he did them, it would have been someone else manipulating and committing atrocities - whether during the 40’s or later on.
Also the UN wouldn’t exist and the world would probably have a forget ton more wars and skirmishes to confront