A kind breakup isn't a vague one. Vagueness feels gentle to deliver but usually lands as confusing or even worse, hopeful, leaving the other person parsing your words for a reason to believe it isn't really over. Real kindness is clarity delivered with care — saying exactly what's true, without using it as a weapon.
Pick a time and place that gives the other person room to react honestly. That usually means in person or on a call, not over text, and not somewhere so public they have to hold their face together. Lead with the decision, not a long preamble that makes them brace for something worse than what's coming. “I've decided we shouldn't be together anymore” is clearer and kinder than ten minutes of throat-clearing.
Be honest about your reasons without turning the conversation into a performance review. You don't need to catalog every flaw to justify your decision — wanting different things, or simply not feeling what you used to feel, is reason enough. Piling on specifics usually says more about your need to feel justified than anything the other person needs to hear.
Let them react. Don't argue them out of being upset, and don't rush to comfort them in ways that blur the message. And once it's said, resist the urge to keep talking just to fill silence — sometimes the kindest thing after honesty is space.