Actually we don't really have the information to judge their relative 'mass murderer' tallies. Van is the central character so we've seen all of his actions. Heinz is a secondary character whose actions we only see on occasion for effect. We have no way of knowing how many living beings Heinz has killed on his way to S Class. How many villages or clans he's eradicated on a 'quest' from some local noble or the local adventurer's guild. You can make a good case that Heinz has been (and on some level continues to be) involved in attempting multiple genocides. When it comes down to it, though, only one of the two is willing to accept that what he's doing isn't some attempt to reach some mythical 'greater good', but instead accepting that his actions are simply perspective. There is nothing so dangerous as a person who has the absolute belief that what they are doing is right and their end goal is worth any sacrifices they deem necessary to achieve it.