Dave Stackhouse also says Walt wasn't himself. They got into an argument after his old friend missed his son's birthday party. That never happened before he won all that money. Megan realizes that Walt had a tumor on his adrenal gland causing his system to flood with hormones which made his moods erratic. The guy didn't change because of the money. Walt changed because he was sick.
Sam does some pretty nifty bike riding during the pursuit of a suspect driving Walt's missing motorcycle. The driver is the victim's event planner, Justine, who was also the instant millionaire's lover. But she's no killer either. Megan finds a knife fragment was carried down the aorta and lodged in the femoral artery. The epoxy glue found on the blade leads the team back to Dave Stackhouse as a suspect.
Dave denies killing his friend. Megan realizes that he suffers from a pinched nerve that comes from hunching over a welding torch for years. Dave didn't have the arm strength to make the fatal knife thrust that killed the victim. Bud and Sam aren't convinced. A bruise on Dave's midsection verifies his story that the victim impaled himself as his friend was trying to control his erratic behavior. Megan says Dave was the only person who cared about Walt enough to realize that something was very wrong. Case closed.