Quick thoughts
If Seiichi did what we were shown, then that sucks and just completely betrayed what we were led to believe up to this point
I'm never going to forget the fact that Fukiishi didn't directly answer whether she liked someone else or not, so yeah...
If the author really does want to go the whole "he had a manic episode and did that" route, then he better say at some point that Seiichi has actual brain damage and a hereditary mental disorder along with being a descendant in a long line of natural born murderers (which i genuinely think might be the only plausible case here)
That concludes my ted talk