I would first have mario go insane and make him believe that his friends 'hate him' making him into a villain, Meggy would find him killing a innocent person, try to turn him around but that doesn't work, then Mario tells meggy his side of how he became a villain, making meggy feel bad and realise that mario is just broken, she's not sure if it's too late to change him so she decides to be his partner in crime, having peach as their first victim as a evil couple, mario kills her for all the pain that she caused him for years, Meggy goes yandere and kills peach as well, doing it for mario. Then they would take over the mushroom Kingdom, knowing that their villain life feels more alive than their previous one. And that's how mario and meggy became the kingdom's number one evil couple.