LOL....
I know its not funny.. but knowing what I went through with my nutcase tiger barb, I can only imagine what is going on in your tank!

it seems that some of them can be insane and not get along with anyone in the tank. I had one that went after everyone ...relentlessly !! Gave him a proper name... " lil ba*****"! you can fill in the blanks heheh...
I took him out of the tank and put him in with the betta for awhile. He was scared to death in that tank and stayed in the corner, I said.. there! that is what you get for tormenting the other fish. ( i was in a bad mood...). Eventually he found out that the betta was not going to bother him and it started fin nipping at the betta! So i said.. ohh nooo thats not gonna do!
So i put him in with the discus... they are so much bigger I figured there was no way he was going to pick on them. Well, he didnt. and after awhile he started thinking of himself as a discus and followed them around. lol..
That lasted for almost a month, then I saw him start to pick at them too, so I took him out and put him in the breeder net box back into his original tank where there were two other tiger barbs that were getting along fine with other fish. Well when another tiger barb started bullying up the other tiger barb, I put that one in the breeder box net. All I can say it was a sad sight and before I knew it, the lil B.... actually killed the other tiger barb! I tell you.. that tiger barb was insane!... sad..Well it lasted awhile and then finally died. I figure that it was just a bad batch of tiger barbs, as others were not that mean.. I will never get another tiger barb for my tank, even tho they are great looking fish.
Sorry this was a long response.. just want you to know that in the end, there is not much to do. If you can bring it back to the LFS, you would save yourself a lot of worries and headaches. It will pursue the molly and no telling what will happen in the end. If you get more TB's.. chances are you will have more aggression in the tank. I wouldnt do it if it were me.
Good luck... and do what you have to do to save your molly!
~ kate