You were trying to bite at the time. Quite persistently in fact.

[He did think for a while about replying to this at all, but had reasoned in the end that maybe Mettaton did deserve to know who had put an end to his suffering, even if he did seem good-natured about it.]

The person who 'offed' you was me. But don't thank me. It wasn't anything I want to be thanked for.