Telling people to kill themselves

[miajag]

It is pretty common in my group of friends to jokingly tell each other "kill yourself" in response to some trivial matter. Example:

Friend: "Hey, I was going to meet up with you guys last night but I ended up just staying in with my girlfriend instead."
Me: "Kill yourself."

We say stuff like this to each other pretty much all the time and think nothing of it. However, it has recently been brought to my attention by a couple people that many find this sort of thing horribly offensive. Are they taking themselves and life too seriously or are my friends and I douchebags?