When Shane McMahon returned to WWE, it was to set up a match at WrestleMania 32 over control of Monday Night Raw. The story goes that he has some sort of something on his father, Vince McMahon, in a lockbox somewhere and he was willing to put that on the line if dear old dad would put up control of Raw in a match against an opponent of his choosing.
That opponent turned out to be Undertaker.
The question, then, became why exactly "The Phenom" would do Vince's bidding. When "The Deadman" returned to Raw, he didn't bother explaining himself, not really, instead simply telling the older McMahon the younger McMahon's blood would be on his hands.
In an interview with talkSPORT's Gorilla Podcast, Kane tried to explain it:
"He has a challenge in front of him, and that's what it is. I don't know if there's necessarily a nefarious back story or anything like that."
The simplest explanation, it seems, is the correct one. It's just an employee doing as his boss asks.
Good enough for you?