Well, i can’t speak strongly for 5e mechanics themselves (though looking at the spell and others, 5e seems to have done away with Evil and Good-alinged spells as far as i can tell), but using negative energy to damage someone isn’t any more or less evil then healing an ally, IMO.
Negative energy is a natural force, just like positive energy or fire. Surely evil villains can bind or heal each other’s wounds in combat without threatening to become “good” and “heroic”, yes?
What matters is HOW you use it, not that you do. If you use it to purposefully and knowingly inflict unnecessary pain and suffering, its an “evil” act just like any other kind of torture. If you use it to protect the defenseless and stand up for what’s right, its a “good” act, just like anything else a paladin on another world might do.
Also, keep in mind the usual blah, blah, blah … Dark Sun …blah, blah, blah… survival… blah, blah, blah shades of gray & moral quandaries, etc.
I think it’s fun that your character is having this moral quandary in role play though - super cool.