It didn't show the actual beheading, just at it didn't actually show the disembowling .. it just shows the the giant axe-type-thing coming toward his neck while he's smiling and looking at the angelic version of his wife walking toward him. In the next and final scene, Robert the Bruce, as a narrator, says, "After the beheading, William Wallace's body was torn to pieces. His head was set on London Bridge, his arms and legs sent to the four corners of Britain as a warning. It did not have the effect that Longshank's planned."
Here's what happened in real life, if anyone is interested:
Scottish History.com
Word.