Michael was warm and loving to Kay in '45, but when he came back from Sicily and reconnected with her in New Hampshire in '47 (wearing that ridiculous homburg hat and looking like a pint-sized banker), he might have been negotiating a merger or acquisition with a weaker company--in fact, he wanted to own Kay (listen to the bloodless way he said, "I love you"--it's a wonder she didn't keel over from fright).

His actions during the Tahoe shooting were instinctual (good, protective instinct) but that scene wasn't about warmth--it was about survival. Kay certainly didn't take it that way--after the shooting, when she was clutching Mary, she gives him a look that would kill. It was all over between them at that point.