I don’t have my own copy of TWOIAF, and it doesn’t mention his ultimate fate, but what happened to Erreg the Kinslayer?

Something really, really bad – bad enough that he’s remembered in song as a cautionary tale. I don’t think he wasn’t the Kinslayer yet when he sacked High Heart. My guess? Went mad from the visions of the weirwood stumps and killed his whole family.