Dorothy: Being right-handed, I would slash from left to right. But from the fact that the murder weapon was dropped to the left of the victim, we can deduce that the murderer is left-handed. Notice that Gloria, like most left-handed people, wears her wristwatch on her right wrist. I think it is painfully clear that Philip and Gloria both planned murders, and each meant to pin it on the other. They knew that the lights would be turned off when the cake appeared because that happens every year. Coincidence? Ha! Siblings often think alike.
Marlowe: You've given us a who, a how and a "Ha." But what about why? What's the motive?
Dorothy: When a 22-year-old marries an 88-year-old, chances are she's not after his body.