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.