By itself, I'd assume the performance was so bad the song died in the process. The singer murdered the song. But alternately, a "killer performance" is an amazing performance - so powerful it could kill, I suppose.

It can have several meanings 1, he sang the song terribly, that he killed it(negative) or he sang the song so good that he killed it (positive) . It all depends on the performance.

The song used to be popular. Then this guy sang it, and it was so bad, it killed the popularity for that song.

He totally rocked at singing the song.

If I heard someone say this about a performance of a song, especially a rock-and-roll or heavy metal song, I would think that they sang the song well. If they said it about a classical song or opera aria, I would think the performer sang poorly.