I have done this TWO TIMES OVER and it still doesn't work. This is exactly what I do. I go and talk with Aaron. He sends me to go help Mookie. I go to Mookie at night. Mookie and I have a chat. Mookie see's Lassal. Mookie gets killed by Lassal. Lassal attacks. When almost dead, Lassal has his usual conversation with mem, BUT DOESN'T TURN INTO A GASEOUS FORM! He keeps on attackin. I try to insta-death him (cntrl - y). Doesn't work.

So in other words... Is my game totally messed up or is there a fixer for THIS???

SOMEONE HELP ME!! Please!!!

This happens from time to time, and it's not even very rare that it happens twice in a row. I'm afraid that reloading until it works is your only option - I've never heard of a fix that takes care of this permanently.

So is it almost impossible for it to happen 3 times in a row?

I don't know, never happened to me more than once. I'd say the odds of it happening thrice are quite small. Why, did it just happen again in your game?

Actually in my bro's. :D We found a way around it. You just go and jump infront of the Shadow Theives Guildhall, and tell Aaron what happend and you get your next mission and Lassal is gone :D