Bro

he left to rebuild his country and take over the throne of Theo's country, he didn't leave just for a funeral

lol no, you can tell from chap 61 that Seth really only left for the funeral. He was planning to right come back to their mini hideout and he doesn't know Theo already left. After he finds out he'll probably understand why Theo left and go back to his country. Im guessing Seth expected to raise their child together for a little while and didn't think about what to do next. I can see why you may think that tho.