5-year-old shoots his 2-year-old sister

Sarcasm? That post is a mile closer to a joke than sarcasm. And even if it was sarcasm what would it be trying to point?

and whats wrong about joking about this? do I seriously need to respond?.

While joking about is bad when you only look at the joke. But if you look at from the perspective of how stupid arguments for no gun control are, then the joke has its place. It obviously has more potential for changing things than people who just feel sorry and try to sympathize for the tragedy.