You're on the right track with the guesses in your subject line.
It is a sort of antithesis, by the definition:
Juxtaposition of contrasting words or ideas (often, although not
always, in parallel structure).
I'd say it's more precisely an example of enantiosis:
Using opposing or contrary descriptions together, typically in a
somewhat paradoxical manner.
Money is an excellent servant but a cruel master.
I could neither continue listening nor turn away.
Blessed is the man that walketh not in the counsel of the ungodly, nor
standeth in the way of sinners, nor sitteth in the seat of the
scornful. —Psalm 1:1
All definitions and examples are from the Silva Rhetoricae.