"Sealed". Meaning MC CHOSE not to kill him, for whatever reason, and let's not even pretend the author ever considered having MC pull a Kars on him if he truly was unable to finish the job.
Settle in folks, we're in for another long and pointless arc. My money's on either the guy being some form of relatable, or another case of past MC being a fickle pain in the ass and not doing what needed to be done.