Jason Voorhees has never been easy to kill. There are times when we have been so sure he was dead and it turned out he was actually alive (and still coming to kill you). Voorhees has not changed his ways in Mortal Kombat X; even when you think he's dead, he just gets back up again (it's like that Aaliyah song).
In an apparent glitch, Voorhees is getting back up after a Brutality is done on him – even if he has no legs or no head.
The Mortal Kombat co-creator Ed Boone is known for sneaking fun things into the game, like unleashing grandma fury as a Brutality or even modeling Voorhees' face for no good reason — it's under a mask! So, whether or not this is a glitch or paying an homage to the homicidal maniac is unclear.
So, who needs legs?
While you're at it, who needs a head?