Skeppy can change his skin to diamond to protect himself, diamond cannot melt because of lava.
So when Bad pushed Skeppy in he tried to activate his powers to protect him but only was able to cover half his body before the rest hit the lava.
When everything is said and done half his body is burned and the other is completely fine.
Skeppy won’t be able to remember how or why this happened but Bad does, Skeppy is a walking memory to him. A horrible walking memory.