It’s the almost that gets him.
It is ALMOST against his conscience.
If it were ACTUALLY against his conscience, he’d switch out his poisoned bated sword and forget the whole plan.
But it is only ALMOST against his conscience. He’s hedging. He’s close to making the switch. But he’s also not listening. Claudius has said “I do not think’t.” He has an out. His king is (possibly) expressing a hesitation which would give him leave to extract himself, from this mess. But instead he has this ALMOST moment, ALMOST a conscience, ALMOST a moral question.