My Hawke didn't spare him out of some dream of happily ever after. If anything, every second of downtime between fighting is spent coming to terms with all the degrees of horrible that their lives are going to be.
My Hawke spared him because she had spent the past six years with him by her side working towards a better solution. He appreciated her efforts to reach out to templars, he agreed that there were some who weren't horrible. He was willing to entertain the idea of working with the Chantry, and he was hoping up until he put the bomb in the Chantry that Elthina could finally be persuaded to do something.
But she didn't do anything. By the time Hawke helped Anders, and talked to Elthina about the situation, she realized that this woman was going to do nothing to help the mages. She was going to nothing to help the templars suffering under Meredith. And she was going to let it keep going, in the name of balance that wasn't balance at all because one person was clearly wielding all the power, and wait for the Maker to decide.
As far as Wil was concerned, Anders' act was that sign Elthina was waiting for, what Flemeth had foretold, and something that was horribly, horribly inevitable. She knew the moment she saw the explosion what side she'd be on, because she'd spent her entire life there without even realizing it, and there was no way that she was going to let Anders go because he was her responsibility, nor was she going to kill him because she knew she'd need him in the battles to come.
And she loved him. And she knew exactly why he'd done what he'd done because she was coming to her wit's end with the issue and she was just a sword for hire turned quasi-politician. She had no spirit driving her, or any memory of fear or abuse to draw on. She just had her lifelong fear of losing her family to the templars, her experiences in Kirkwall, the dead-end conversations with Elthina, and Meredith's own madness staring her in the face. She might have been pissed (she was sooooo pissed), she might never be able to fully forgive Anders, but she understood.
She also knows that he might still be dangerous. The hope is that it was the stalemate that drove him to act, and once things are set in motion he'll go back to a more tactical approach. But if he doesn't, if he wants to escalate...she will have to kill him.