Take the context clues from last chapter , Seichi isn't literally dead, but inside of Seiko's mind seichi has been 'killed' off and is apart of the rest of her dissociative world
Welp that’s it, dropped, translators I wanna KISS TF OUT OF YOU, you all did a great job in the end but this damn manga/mangaka has gotten so wishy washy and that’s my final straw, started great, got too artsy and now just a clusterfuck of bordem
I think this when she tried to kill baby sei as we saw before in Seiichi's buried memory. But maybe the reason why is because she saw sei as an extention of herself and was trying to use him unlock one of her barrier memories all while dissociated and losing control of herself. I dunno but it seems like it's being implied that when Seiko was young would come to that same spot to see the view, she tried to take her own life. But she had blocked that memory out. So when she was reminiscing with Sei the memory of her suicide attempt started to creep back up, but she couldn't fully process it in her distorted dissociated state. So instead she recreated, but eith Sei since at least at this moment she sees him as something that can't be severed from herself. Something that was always apart of her and will never not be.
My assumption is that she's using baby Seiichi as a stand-in for her childhood self. Signifying that since she "doesn't know" and it's "so far away" that she might as well kill those crappy distant memories she's barely holding onto. I imagine this will lead to her just slowly unraveling mentally in the background of what we've already seen all because she truly feels empty after this moment