The human animal is a beautiful and terrible creature, capable of limitless compassion and unfathomable cruelty.
This is taken from the lyrics Sophia by The Crüxshadows. It had stuck with me for so many years and wondered if there's a literary device to name it or a figure of speech.
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.
Examples:
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.