Why did the Scots hand William Wallace over to the English when he was considered to be a Scots hero ?

Probably because he was becoming a liability.  Every time he made trouble,  Edward, Hammer of the Scots,   would assemble a huge army, walk them north for a week or two, and then give the opposition a bloody good thrashing. Eventually, the Scottish Lords decided that Wallace was becoming someone of a nuisance , and handed his over to the English who had him hung, drawn. and quartered, not a nice way to go.

His last thoughts wold  probably be  the realisation that it was always a good idea to know precisely who your friends were.

The movie Braveheart portrayed Wallace as a nationalist Scottish hero.  But this was centuries before nationalism, in the modern sense, developed.  The wars in Scotland, as with most wars of occuption and resistance, weren't a clear cut thing with the evil invading English on one side and the valiant unified Scots on the other.  There were Scots who supported King Edward I and it was one of these lords who captured Wallace and turned him over to the English for execution.  (A similar thing happened hundreds of years later with Joan of Arc, who was captured by a French faction loyal to the English and turned over for trial)

Money. I know this to be a fact. Most of my ancestors were part of the handover. Yes, he was a hero, and we all really liked him. But for the kind of money they were offering....... well, let's just say you never look a gift Englishman in the mouth. My kin said to Wallace, as his captors were dragging him away "Hey, Wallace, it's just business" and he said "Ah, right. These things happen". So, in the end, everyone was happy. Super!

Because Scotland was under harsh English military occupation. Wallace's rebellion had failed by then and the English were hunting him everywhere for revenge. They would pay a fortune to whoever turned him in and Scotland was full of traitors and collaborators willing to take the money. One did, a Scotts traitor a local sheriff working for the English turned him in return for the money lands and title. 

P.S. The creeps name... John de Menteith...later he turned traitor again, betrayed the English joined Robert the Bruce/Scotts and got paid for that too

The Scots have always had a problem with traitors that would try to curry favor with the English.  The English were very good at exploiting this.