IMHO (forgive me if I ramble, haven't had my first cup of coffee yet, but I can smell it brewing) he killed the children first. Not just to be rid of them, but to punish SW. I also believe he showed her what he'd done. At that point, sick and tired, she would either attack him or collapse, I believe she was totally over-whelmed, collapsed, and was easy prey. She may have wailed and was heard by the neighbors who thought it was the dog, Later the dog did cry and the neighbors decided all that they heard was the dog. I believe the punishment thing fits; he puts her in a grave separated from her children and the children are placed in separate tanks.