It's less a personality switch but an impulsive reaction. YT can't control his impulsivity bc of the fragments in his brain, it's making him violent and brash. Add to the fact that SH drives him crazy in a good and bad way. He's angry at SH and feels betrayed bc he's broken his promises twice now, so yes, he's mad and rightfully so. He's still soft on SH bc he loves him so much he doesn't know what to do with himself. He can be flirty but when he gets the affection he didn't expect, he gets flustered and shy because he really likes SH. he doesn't know what to do with himself so he runs away.
In that respect, him and SH are similar bc SH ran away from YT bc he wasn't ready to admit he liked a man. These affections would become a liability in his line of work, so one of them had to go and it was YT.
wtf is happening here omg?! yuta's personality switches are seriously giving me emotional whiplash and cloud my comprehending abilities. does he like have bpd or he's just a sociopath?