How is Invader Zim "Evil" as some Christian groups call it?

I love Invader zim and johnny the homicidal maniac (both by Jhonen Vasquez) but i heard that a Christian parenting group was one of the driving forces behind Zim being canceled ,first many people told me that doing martial arts would send me to hell , now people are saying the same thing about me watching Zim and reading JTHM ,I'm a Christian and i understand that Zim and JTHM are dark , but they're funny too and they shed light on what's wrong in the world , a few times when i got depressed id watch Invader zim on DVD and i would feel a lot better at a time when prayer wasn't doing anything , so I'm just curious what is wrong wit loving zim and jthm and will that hinder me from getting into heaven or anything ?