Just playing the game to find the weapons fatalities, Ill list them here as I find them. Note: you have to unlock them per character before you can use them. Here is what I have so far.

Johnny Cage: Up, Down, A
Reptile: Up, Back, A
Nitara: Down, Down, B
Hsu Hao: Up, Forward, B
Bo Rai Cho: Down, Back, A
Quan Chi: Down, Forward, A
Sareena: Down, Up, A
Drahmin: Down, Down, A
Mavado: Down, Back, A
Shang Tsung: Back, Back, B
Scorpion: Up, Down, A
Raiden: Up, Down, B
Cyrax: Up, Up, B
Sektor: Down, Down, A
Noob Saibot: Up, Back, A