Responses
Nah, Seth didn't kill the boys because of Horus (back then he didn't even know Horus existed), it was because he was trying to find Anubis who had run away, and because Anubis wasn't a god he couldn't sense him so he had to check kids one by one, and got frustrated every time they turned out not to be Anubis, so he killed them.
Ah, right, I forgot about the fact that he just recently learned about Horus' existence.
Was he trying to kill Horus? Because if he was, it's kinda ironic that he mistook Horus with Anubis...
Still, the situation strikes me as odd, somehow. That he would go searching for Isis but end up destroying a temple and killing children... Present Seth doesn't seem the least bit interested in killing Horus, so why would past Seth just kill a bunch of human boys? Also, Horus says that the temple seemed to have been destroyed by war. It's kinda unclear if he's referring to Seth himself (Seth is the god of war) or if it literally looked that there had been a quarrel between two factions. If it is the former option, who fought with Seth? And could that individual be somehow involved in this killing spree?