I agree! since it is suggested that at the time someone else knew yuuga was gay and maybe that person either couldve been same age perhaps and had either feelings for the uke or yuuga and acted out of violence due to jealousy or malice or i might be wrong maybe he himself was gay and couldnt accept that or hated that about himself thus it manifested into violence and the first incidence wasnt as calculated as it may have been impulsive and out of pure anger then the later cases are just him reenacting acts of the past (his killing pattwrn). Or my prediction may be off by a landslide, but the councillor for sure seems to be the murderer
Targets gay students (15 yo)
Knows his targets very well (Accrdg to the seme, the culprit knows when his victims will be alone)
In the characters introduced so far, I think the student counselor is the culprit.
He goes to different schools which gives him a chance to meet potential victims with no connections at all.
So how does he choose his victims?
In a perspective of a 15-yo boy who finds himself being attracted to same sex might be confusing for him. The student might seek for assistance by asking for an advice to the counselor. Or the school conducts check-up with students. With that, he can easily obtain basic info of the student (name, age, address etc.) and choose a victim.
Well, I tried. This is my hunch. :)