Connor did kind of mutilate all of his elven servants before killing them, killed off a good portion (if not all of) the people he owed his noble obligation to and ruined my City Elf Warden's shoes.
I mean, the Warden has slaughtered armies for less, so I don't see why we're suddenly drawing lines all of a sudden.
I'm not saying that there aren't reasons for killing him (though by your logic, we shouldn't spare almost anyone of the major NPCS we had the option to, not to mention that we are forced to be mass murderers in Bioware's games)
What you said is true, but it wasn't completely his fault. He had no decision in his mother's actions of hiding his nature, he didn't know much about magic (and even less about demons) due to his crappy teacher, and a kid isn't exactly mature enough to discerne what is right or wrong (the demon that offered him help). Jowan and Isolde are more at fault for what happened than him.