AMC: How can Erica forgive Zack for killing her son?

That story always bugged me because it was done so coldly. Zach shot Josh, and immediately said to grab his heart! What bugged me more is that Kendall never left him for it and that Erica of all people didn't go after Zach with both barrels! Since when does anyone mess with one of her kids and gets away with it? The writers didn't handle the story well in my opinion. They would have handled things WAAAAAAAAAAAY different had Bianca or Kendall been killed. And the Erica I know would have moved hell on earth to keep Zach away from the kids after doing what he did. He should have gone to prison for killing Josh.