Variant of kthxbye and kthxbai, but with a mordant
edge.
The original term kthxbye (okay, thanks, bye) is generally used to terminate a (n
online) conversation in a quick manner, often with slight
insult to the other party. The implication is that the other person is not even worth talking to.
kthxdie is like that, only more so, and with an added death wish.