
Originally Posted by
You
Its a little more narratively contrived than by chance.
Murder was losing its luster for Ryuunosuke so the prodigal son decides to return to the home of his first murder victim, his sister. In the storehouse where he buried her body he finds the spellbook heralding from HGW2 (the implication here is that his ancestor participated in that Grail War). Incorporating the rituals in his murder makes the killings feel interesting again, so he continues doing it, which is how he ends up summoning Gilles.