It was a curse fallen onto him.

Anyone he loved will always disappear from his grasp, sooner or later. He hates it.

And by that, he refrains himself from getting attached to you right after the moment you showed kindness to him. Because he knows if he's careless,he'd lose you too.

But he was stubborn.

'It'd be just this time'.

Hence that's why it shattered his soul when he sees you lying against the ground in your own pool of blood, slowly dying—presumably just escaped from infinity castle.