The most recent trailer for the reboot of Mortal Kombat explains why Scorpion hates Sub-Zero's guts, but it's pretty standard revenge stuff.
You might think that there is some huge reveal to the recent trailer from NetherRealm Studios about their upcoming Mortal Kombat. I mean, the game is a retelling of the first three MK games after the thunder-god Raiden sends a message back to his former self. We will finally understand why Scorpion, Sub-Zero and Lizard all look similarly and why the undead Scorpion has it out for his blue counterpart. But really, it's just because Sub-Zero killed Scorpion and his family. Sorry, that's all the writers could come up with for this one. Dead family, revenge - ho-hum.
Perhaps I'm being wrongfully hard on the designers, but I think that I'm bound to be underwhelmed after seeing the awesome fan-made Mortal Kombat trailer. If the remnants of Midway haven't hired the guys who made that short film to write and create the cinematics for the new game, then the powers-that-be are doing the gaming world a great disservice.
Then again, maybe the filmmakers behind that film are working on a Mortal Kombat movie that doesn't suck. That would be OK, too, I guess.