Slight movie spoiler.
My thoughts on what Natasha whispered to Clint right before Thor took Loki back to Asgard.
It gave Clint a great deal of satisfaction to see the psychopathic bastard restrained, with his lying mouth sealed shut. He didn't know what awaited Loki in Asgard, but he was sure it wouldn't be pleasant. No matter what his punishment was though, it couldn't make up for what the murderous bastard had done. Not just to Clint, but to the world. So many had lost their lives. Clint himself had been unmade and reformed into a good little lackey, using his assassin skills against innocent people and former coworkers. He had killed agents that he used to work with.
He stared hard at Loki, willing the bastard to go up in flames and die in writhing agony. Of course, that wouldn't happen, since he was almost impossible to kill. Hulk had unleashed on him, and he had only a few cuts to show for it. It was disheartening that such an evil bastard could be so indestructible.
Natasha sensed his dark mood, and moved to his side. She cast a coy look back at the fallen god, and leaned over to place her mouth close to Clint's ear. "There's a man who should indulge in a little self love if he can't get any. It might help these periods of acting out."
Clint grinned nastily, hoping the bastard could read minds. "I think self love is the only kind he's going to get for a long, long time."
Natasha smirked. "If they have the equivalent of prisons, he'll be bent over someone's bunk soon enough. He's a pretty boy."
"Then he'll know what it's like to be dominated."
Still, Clint felt a tiny bit of compassion at the haunted look in Loki's eyes as the two Asgardians disappeared. Even villains had a story to tell about why they did the things they had done. He imagined that Loki's story wasn't a happy one. Did it excuse what he had done? No. But it probably went a long way toward explaining why he had done it.