Franco was a psycopath and Mark knew it from his previous experience. He was very violent even to people who had done nothing wrong to him.
Mark had crossed Franco stealing 16.000 pounds from him1.
Just the two above are enough for Mark to know that Franco was not one to forgive or forget about what had happenned.
On top of that, it is assumed at the end of T1 that Franco was going to go to jail due to his outburst when he found that Mark had stolen him the money. Remember that Franco was already being wanted by the police for a robbery, the obvious implications of two policemen arriving at the scene would be that he would be identified and jailed2.
Since then, it is assumed that Franco has been going in and out from prison until he got his definitive sentence. And we know Franco is always keen on blaming someone else for his "missfortunes". Mark not only owned him the money (and all of the imaginary profits that Franco could think he would have made with it) but also jail time.
And, if Mark needed any remainder, the way his "friend" Sick Boy reacted when he came back would have been more than enough warning of how someone like Franco would react.
Yeah, technically they were 4.000 pounds from each, including 4.000 pounds that were supposed to be Mark's own money. But one of the reasons for Mark stealing the money is the -rather founded- belief that Franco was not going to share the money.
2And that would be ignoring the distinct possibility of Franco assaulting the police officers as soon as they entered the room, something he was very capable of doing.