powerless, and out of ideas
It isn’t rage or the will to harm so much as just no other ideas for keeping the bastard from offering something Gately knows that he’s powerless at this moment to refuse.
Context
The white-knuckle test resolved by a fist because nothing else is left: Gately crushes the doctor's testicles not from anger but because he knows, with total clarity, that he is powerless at this moment to refuse Demerol if it is named — and stopping the words is the only defense he has. Powerlessness is not an abstraction here but the concrete reason for the violence; the admission the whole program is built on becomes, in the crisis, the thing that has to physically prevent the offer.