"You kill it!" means that you killed an unseen monster and leprechauns
teleport after their gold-stealing attack, so there are 2 possibilities:
1. You killed the leprechaun with your passive cold attack after it
teleported, meaning the stolen gold is somewhere on the level. This
may be a bug (passive attacks triggering after teleportation).
2. You didn't kill the leprechaun, you killed another unseen monster.