In fairness, Heeseong thought it was his best friend among the clans whom he can trust and he was really attached to the fighting dogs who values loyalty (more than the wolf clan of Chiyoung). The novel was clearer with his thoughts than the manhwa, but then again, I'll wait for MoonflowerGemScans version which seems to take the novel into consideration when they so their translation.
Also potential SPOILERS...
This experience was the incident that really broke whatever lingering attachment he has with the fighting dog clan, even though he would feel the occasional guilt and yearning.
And more importantly, the bitchy ex of Chiyong will get a good beating fr Heeseong later (๑•ㅂ•)و✧
The lil poodle never listens...... （￣へ￣）