We can't really go into exactly how and why this is so cruel yet, but I still want to talk about it because it's insane how little perspective Knives has here:
We're shown a page of Vash being medically examined and experimented on in July (at Knives' behest, while he just cooly watches on), entirely against his control, where hes treated as an object and not a person
And after what would be a frankly horrifying and humiliating ordeal to go through, after having his body changed against his will to check his aptitude as a tool of violence and force it to activate for Knives' will, Knives has the gall to just...say this:
Yet he's doing the exact same thing! He's torturing his brother in cruel and humiliating ways while looking him in the eyes!!