Why does Maggie shoot Phillip in the novel, Hide & Seek?

Maggie makes the decision to shoot Phillip only after suffering at the hands of his abuse for some time. Maggie was already quite sure that he would kill them when he found them, and he'd been raging for some time already. This was the worst he had ever been, and when he finally discovers them under the porch, he takes out a gun and fires at them twice. Only then did Maggie pull out the gun she had hidden and fire back.