"The tesseract – or – your brother's head. I assume you have a preference?"
"Oh I do. Kill away."
It's what Thor would want him to do. Mind racing he stares at the only thing he has left in the universe...
...neither falling from the bifrost into the abyss, through space into a different dimension,
nor being tortured by the mad Titan,
nor SHIELD shooting at him, Coulson firing a rocket launcher at him, catching one of Hawkeye's explosive arrows or being Hulk smashed,
nor a sword through his torso...
...was as painful as watching Thanos press the Power Stone to his brother's head. He's not prepared for that kind of pain. Unable to watch any longer he yields.
"Alright stop!"
Head bowed he does as he's told. Summoned from thin air the tesseract appears in his hand.
"You – really are the worst brother."
That is the most painful thing that ever happened to him.