That has yet to be confirmed.

But, bottom line, Kabal joined the Earthrealm warriors because Raiden sent him a vision to join the other Earth warriors. Kabal wanted vengeance on Shao Kahn and his forces for what they did to him (cutting him up so badly he needed a respirator to live), so he decided to join up.

I think maybe being with them, and knowing what a true fight it was, convinced him that the Black Dragon were a bunch of useless thugs, and he didn't want to be a part of them anymore.

If his ending in MK3 is true, he's pretty much Batman now: hiding in the shadows fighting crime. But, evidently, he and Mavado tangled (which would make sense, since if he's fighting crime, he must've come across the Red Dragon, not to mention the fact that he knew about them before since he was part of the Black Dragon) and now he's MIA, but as we can see in MKD, he ain't dead!

We'll find out what happened soon enough.

I wouldnt really consider Kabal a "good" guy. Hes more of anti-hero type now. Hes not a Black Dragon member anymore either, so i'm thinking that not only would he be on the hunt for Mavado, but he would also want other Black Dragon members gone, such as Kira, Kano, ETC.

Like PsychoFight said, Kabal is like a Batman now. Purely fighting crime.

It's a dark night, there's a bank robbery in progress...Time for the Kabal signal!
*light shines on the clouds with the MK dragon symbol*
Kabal: "Time to get to work!"
*crazy 70's BatMan theme song in the background*