If you are an Earl, and you are sent a safe conduct pass to go talk to the King, you're safe, right? You can go meet them, and calmly discuss that alliance you made with a couple of other noblemen, one that is not in favor of the king and his kingly position. Calmly, yes, and then you can go home. Unless it's 1452, and you're in Scotland, and you're one of the Douglases, and the king is known for having a very bad temper. In which case you might get stabbed 26 times and thrown out a window. Really, given Scots history before that, one might have been able to predict that; noblemen getting stabbed despite their safe conduct passes is sort of a theme.