It's listed in his bio that Quan Chi offered him life if he did so.

and..scorpion only thought the original sub killed him.

he had no idea who killed his family until quan chi told him it was the other sub-zero

you mean he when quan chi lied to him about who killed his family. Alright that really helped. Thanks!

I think it went like this

Sub had Killed Scorpion at First because he was from a Rival Clan, then when the First MK had Happened Scorpion had Came back And Killed the Original Sub-Zero, When the 2nd MK had Started, Scorpion had Saw Sub-Zero Enter it so Scorpion followed him (i guess he only THOUGHT he killed him), then in MK4 Scorpion Beat the living shit out of Sub then Sub told him that he Didnt Kill his Family and Clan (so i guess the Original Sub-Zero had Taken part in the SLaughter of Scorps Clan) then Quan Chi told Scorpion that it was him who Killed his Family... so NOW scorpion is Neutral to Sub.... damn that was a Mouthful..

mk4 is not really new tournament because it goes way back before first mk was created.the battle between scorpion and subzero was told in mortal kombat mythologies.also i've noticed the subzero in mk4 has a scar like the one in mk3,too confusing to say,but i think that mk4 was in the story in mortal kombat mythologies.is that the answer you're looking for?i've been a big fan of mortal kombat for 10 years,i've played all mk games already.ttyl

OK, MK4 Sub-Zero is the one from MK3, MK1 sub Zero is dead... He was back in UMK3, but even then it is not confirmed what part he played in the story... I think that Scorpion was after The younger Sub-Zero in MK4, probably because Quan Chi simply lied to him, saying somehting like "While you fought with Sub Zero in the shaolin temples, his younger brother was the one to go and slaughter your clan." That's the only explanation I got, but the bottom line is, Sub Zero in MK4 is the younger one, who only came after MK2... Simple enough?

Yes that is pretty much the way I see it too, Sub-Zero from MK2 is Sub-Zero from MK3 is Sub-Zero from MK4 (and quite possibly onwards, until Midway decides he dies I guess).

But what I got from Scorpion wanting his head on a wall in MK4 was that he thought it was the original Sub-Zero so he decides to go after him again. Thats what I thought, I aint sure if it states that there but it made sense...to me anyways.