The way the lore portrays it, there is no genocide or murder in Stratholme. They were already dead, they just didn't know it yet. I'm a loud and proud Nelf and soon-to-be Worgen :), but Jaina and Uther abandoned Arthas. Was it easy or fun for Arthas, no, but was it necessary, yes. Even the Bronze dragons say as much. The real change in Arthas was what the other two did in leaving. Stratholme didn't change him because of the killing the proto-zombies (even if some were temporarily young or female), but because his closest friends and allies, his mentor and his sweetheart (kinda), were unwilling to find a better way to really help Arthas. You may not emotionally like what Arthas had to do, but that does not make it any less necessary. It certainly does not make it wrong.

On another note, it seems to me that Uther and Jaina were cast as heroes before the Culling was ever conceived; even in Vanilla they have always had that aura about them. This event is perhaps the only (so far as I know) testimony from their past that would besmirch that aura. While I find their abandonment reprehensible I also realize this is an event that happened long ago, and while Uther did not live too much longer, it is obvious that this created a scar upon the person and psyche of Jaina that exists today. Look at the Battle for Undercity. Instead of running away, she takes action. While I would not mind seeing the Horde taken down a peg or two :), I realize that Thrall is probably our best hope for a future where both factions are moving toward peace. She learned from her mistakes and grew to be a better person. That is why I can still consider her a hero.