And he could. He could. That's the thing. All it would take is a slightly too-hard flex of Luther's thick fingers, and he could pinch Diego's aorta shut, collapse it. Snap his spine. Or simply press inexorably harder and harder until he suffocated all the breath out of his brother, like he had done to Vanya—

Luther's aware of how little it would take. Each iota of force he has at his possession, and knowing what amount would kill an enemy. How many years he's carefully-honed his control and strength, so that there's rarely a slip anymore, not a loss of that tight rein he has over himself.

And yet. How easily he could let that all go, and instead simply pull the other man apart. How fragile human bodies are. They're just a loose collection of sinew and blood and bone. He could crack it open, spill that blood on the floor.

The thoughts come automatically, just like their father trained him to — those ruthless calculations, how to apply that strength to most lethal force — but then through the ringing in his ears, Luther hears that word monster and he stops instead. Shoves Diego against the wall, driving the breath out of him, but then drops him back to the ground. Luther's expression furious, but also— confused. Suspicious. They've fought before, they always fight, and yet...

"Something's up with you," he says.