I feel like the word "evil" is only coherent under absolute morality (which I'm pretty sure is wrong) and falls apart trivially under relative morality. Just because it is regular used as a metaphor by the population doesn't seem worth the linguistic precision lost by a concept so widely misunderstood.

Do you mean absolute or objective morality? The two are not the same (see the WP articles).

I would be interested in hearing you more clearly define the nature of evil as negative utility

The commitments you have already made, if true, make that impossible. Utility is clearly relative. You say Evil falls apart under relative interpretations. Therefore, there can be no interpretation of Evil in terms of utility where it doesn't fall apart.