Explain what finally persuaded Maxim to kill Rebecca.

Maxim kills Rebecca when he learns that she is carrying another man's child, a child that she throws in his face and plans to pass off as his.

"God, how funny," she said, "how supremely, wonderfully funny! Well, you heard me say I was going to turn over a new leaf, didn't you? Now you know the reason. They'll be happy, won't they, all these smug locals, all your blasted tenants? 'It's what we've always hoped for, Mrs de Winter,* they will say. I'll be the perfect mother, Max, like I've been the perfect wife.

And none of them will ever guess, none of them will ever know." 'She turned round and faced me, smiling, one hand in her pocket, the other holding her cigarette. When I killed her she was smiling still. I fired at her heart. The bullet passed right through. She did not fall at once. She stood there, looking at me, that slow smile on her face, her eyes wide open ...'