Do you really not understand?
Let me explain this to you in a clear manner:
Sometimes when I leave the apartment I'll tell my sister something really rude as I leave. Like Idunno, "I hope the apartment burns down with you in it so I'll never have to see your ugly face again." This passes fine as a joke because we literally grew up together. She will probably respond that she hopes I get hit by a truck. There is an unspoken mutual agreement that "fuck you" means "I like you, mate". Because when we use that kind of language to each other and it's consistently taken as a joke, it is a further indication and reminder of our close friendship.
The kind of joke you made would have passed if it was made between me and punkarsenic, as we get along well enough and relate on this specific topic. Same goes for a lot of other violent, heavy or political jokes.
With a more shallow friend, it could have passed if it had already been established in the immediate discussion that they relate on some level.
When it comes to someone you are not friends with and you disagree with on a regular basis, violent jokes and jokes relating to death runs the risk of coming off as snide, ill meant, or worst case scenario threatening. Yours was small so it came across as snide.
If you had worded it differently or established some kind of relation to the topic indicating you are in the same boat or get the struggle, it could have passed. It wouldn't have been funny, but it would have been clear what your intention was.