based on this there is like so much wubble angst potential right?
this is probably early on in the osmp universe so there weren't very many people in the valley yet; for the sake of the story I'll say it's just sbi and shelby plus maybe sneeg, ranboo, and tubbo (they aren't important idc)
so my idea is that Wilbur and Shelby were maybe friends... probably something a lot more and Wilbur got reminded of Shelby's not ghostyness some way or another. he panics, and makes a plan. (the plan being "if I kill my girlfriend before she dies then maybe she'll be a ghost too!" This is not a very well thought out plan. Wilbur is stupid.)
I don't imagine he'd have wanted it to hurt her very much so it was probably some kind of poison rather than stabbing or anything? like he took her out on a picnic (up on the same mountain shes buried on later because Wilbur is poetic like that n shit) and slipped something into her food (he's a potion maker so it was probably something to do with that)
he buries her in the mountain and comes home without Shelby. Phil has a sneaking suspicion he knows exactly where shelby is and that Wilbur doesn't regret his actions at all. (he's right)
Wilbur was that one to tell Tubbo where exactly to dig in the mountain. He was the first to greet her and show her around with her new amnesia. he shows her the plot of land right next to his house.