I still don't get why seth killed so many children and people? I know that seth was a victim of osiris but what does that have to do with killing his people?
He killed many of them in order to be able to make his weapon stronger and be able to defeat Osiris, as he knew Osiris being dead wouldn't stop him and he would sooner or later find a way to keep stalking him, even from Duat. The reason the children from Kemis, in particular, were killed, was because Seth was desperately checking every single one of them to see if they were the runnaway Anubis, and like the comment above said, he grew frustrated when they turned out to be some other kid instead. It's been stated before that gods can't sense Anubis' whereabouts (nor Horus') because those two aren't gods themselves yet, and gods can only use their godly senses to find other gods, not humans or anything else below. So Seth made the humans bring him boys to see if they could get lucky enough to find and bring Anubis to him mixed among those other kids, grew desperate when none of the kids turned out to be his son, and killed them out of frustration. Horus was ony spared because Seth was already drunk and high enough to mistake him for Anubis.
Anyway, the part about Seth killing humans in order to use their souls (particularly the souls of women and children) to make himself a stronger weapon to fight Osiris, is said in chapter 36 (around the part where Osiris says to Seth, "To think you would turn souls, and not just bodies, into weapons") and in chapter 39 (around the part where Seth tells Isis, "Thanks to you, I was able to (I came up with a method to) make a sturdy sword", and he mentions how he used the souls of those women who died with feelings of betrayal towards their men who had betrayed them in order to sell them as sacrifices to fulfill the request of such an offering from the god of war, Seth). Anubis being able to come and go undetected by the eyes of the gods (mainly because he isn't a god himself) is said by Horus in chapter 26 (though IIRC, it was said other times as well... and let's not forget how Horus is always able to sneak up on Seth, undetected, when Seth's distracted, that's because Horus isn't a god either, so Seth can't sense him).
He is always irritated about their relationships. Last episode also he didn't let them do it in the office. Also this episode he is irritated because kyung joo visits woo young everyday?