Imagine:

You had known Loki since he was a little boy (which really meant that you'd known each other for thousands of years), and had since become close friends. 

Unlike most, you justified Loki's actions in New York, sticking proudly to his side, having no shame in pledging your fidelity to the trickster. When you admitted your loyalty to the God, he was more than happy to welcome you with open arms, using a more romantic approach as you two formed a bond stronger than it had been originally. 

He had always promised that you'd become his queen, and for years, you'd believed him, staying directly at his side no matter what hardships he faced along the way. Little had you known, it was all a facade - he hadn't needed your help after all, and most definitely didn't have the feelings for you like he claimed to have had. He had used you; for both romantic companionship, and for the sake of using your trust against you. You had become a pawn in his game, playing right into his hands, just exactly where he wanted you to be. 

The gif above is the smirk he gave you right before you learned of his betrayal, and just moments before he stabbed you out of cold blood and left you for dead.

Yet you survived, and from that day forward, you'd spent your spare time as a part of the Avengers, working the rest of your life to put an end to his schemes.