Hawks wasn't prone to falling for anyone seriously, most times would just be a one night stand with someone and leave it at that. Until he got the mission to infiltrate the League of Villains which he was able to do with ease, he thought it would be easy to deal with the league and be done until he met you.
You were a member of the league that had a similar past to him and you honestly felt like you could confide in him, after a while he soon fell for you the same way you fell for him.
You loved each other and did all sorts of couple things that you could for months. He thought things would be fine that he'd keep you a secret or delay when the other heroes would soon catch the league.
The heroes barged into the hideout and he had no idea how hard things would really be until he saw the look of betrayal in your eyes when the heroes put you in cuffs.
Hawks: "I'm never gonna see her again."
He should've known better than to fall in love with a villain.