Ammunition
Vash hated them. Right now, his hate was almost all-consuming, eclipsed only by the drowning grief they had caused him. Whenever he calmed down, he felt his brother's sick enjoyment; both knew that those two were the reason behind it. Vash raged again.
Intellectually, he knew it wasn't their fault; they were doing what they saw fit, but he still hated them. They had forced him to kill.
They cared for and loved him, and followed him to the ends of the earth. He hated it.
In the end, all their love did was give Knives ammunition.