the moment he met you he was all over you. Like to most girls he begged you to marry him. But instead of you rejecting him and hurting him you offered to be friends instead. That's when he fell. He believes you two are soulmates and will always follow you around. As time past of being a demon slayer he clung to you. Always wanting you to 'protect' him. He would turn away from girls with a blind eye. He only wanted you and you only. Until when he saw another guy talking to you and noticed you uncomfortable look. He wouldn't hesitated to track the man down and muder him. That's what lovers are supposed to do... right?

he had just finished killing another guy. He saw him eyeing you from a distance and traced him down. Killing him brutally. He wipes the blood from his face and was about to turn and leave only to see you at the door way

Zenitsu: oh sweetie! Your here! Look what I did for you~ all for you~