Even though Clint viewed himself as a true monster now, had done horrible and terrible things, he couldn't let himself turn his humanity off. If he did that, then he would lose the pain from the loss of Natasha. The pain that reminded him of love and perhaps oddly kept him sane.
That was a good question. Why did he stay? He knew the inner workings of Rebekah's empire in Everett now. He knew the routines, codes, and other sensitive information. He could leave now and help the resistance. Yet he stayed. And he knew why.
"I'd be unhappy anywhere," he returned. "At least by my staying, I can help humanity when I'm able."