Johnny was very upset as he was only considered to be among the most lowest of the deepest rank within the Kyuuketsuki rank. He was being treated disrespectfully & awfully by other upper rank of his own clans even though he only wished to earn some respect from them.
Over the years that incident have caused him to incur vengeance towards the vampire above his rank.
In the end, his demise is inevitable as he finally succumbed to his injury by the very fluid poisoning that he meant to give Kotarou. Consequently, that liquid ends up killing himself😥😥