A Ruiner is a person who habitually exhibits a desire to spoil or ruin things, often focused on a particular theme. Different from a hater
in that a ruiner tends to hold deep, genuine, and long-lasting animosity toward their target which can reach obsessive and psychologically unsound levels. Ruiners have a tendency to latch on to specific people, whether they are famous or not, and perform stalking-like behavior. Ruiners who are members of a subculture such as a fandom, may paradoxially focus their hatred on their own topic of interest and attempt to throw wrenches in it or spoil the enjoyment for other people.
"Gary just tries to spoil everything whenever people talk about that new band, he can't stand its existence. What a ruiner."