This word originated in South-Eastern Pennsylvania circa 2006.
"Those bapesta kicks are killa sweet!"
"That girl from Georgia is killa sweet!"
The overwhelming power of the phrase "killa sweet" has limited its useage to some extent. Truth be told, there's not much in this world that merits the use of such a word, and its originators have speculated that the world is "just not ready" for it yet. Included below are a few possible examples where killa sweet can be used:
That backboard shattering dunk by Amare Stoudemire was killa sweet!
Winning the lottery is killa sweet!