As I understand it, putting a life-debilitating curse on a person in order to "save them from killing themselves in battle" is not much different from giving them a disability, like cutting off their limb, in order to force them out of war, out of battle in order to "save them."
With this line of reasoning, have you every thought of what would happen after cursing them? "I'm sure they'll be fine, even with a life-debilitating curse, at least they'll be able to live a live outside of battle?"
Before cursing them, why not talk it out? Take time to discuss what each person wants and resolve each other's personal issues. Even if, for example, the person says "No, I won't do it", you can at least renegotiate and make a compromise! There's no be all, end all solution here.
Even if that doesn't work, well, it's better than cursing the other person without their own volition and even knowledge of you doing it to them. You are essentially forcing a choice upon them, one that will ruin their life, all for your own peace of mind.
"It's okay for you to die out of combat from having a literal life-draining curse, but please don't die in combat protecting/in front of me." That's just a shitty and selfish way of thinking.