It was never a promise
"What's between us it's going to stay here, at Hogwarts. It will last until you join the Death Eaters and I join the Order of the Phoenix. When I meet you at the battlefield, you will be just one of the nameless masks."
Those were the words Ginny Weasley promised to Draco Malfoy in his last year at Hogwarts when they were both still free.
"I always knew, I won't keep my promise," the redhead said years later in the middle of the raging battlefield and lowered her wand.
He threw off his mask and raised his wand.
"Avada Kedavra!"