No, I don't think any subject is inappropriate, as long as the idea of specific images and a specific moment is presented in the haiku...
>carotid in hands
>suicide by matricide
>a horrified look
Things like "a horrified look" do not give the reader any emotion or feeling, no do they resonate with the other lines in the poem. "suicide by matricide" is too ambiguous to be useful in a poem that captures a moment. It is as though you are cramming a narrative into one moment, and trying to make it fit. What are the moments in this story, what is horrific that can be told in resonating images and not as a story. What can be implied.
Here is an example of a haiku that might work as "horror". I just came up with this so it's a bit rough (PC) but it might explain what I mean by moments of horror:
a spot of blood
in my son's diaper
Good luck with your own work!