For some reason, it’s very common for one person in the party to want to betray everyone else. Often this goes with the evil PC, as above, but it doesn’t have to. Again, this comes down the core principle of D&D in that your party has to work together and trust each other, or the game falls apart. The problem with betraying the party is that you’re betraying the players, too. The players invest all this time and effort in the party, they’ve worked hard to build trust between the characters, and then that is just torn away. It can cause real-life arguments and fighting. It’s just not worth it. If someone comes to you wanting to have a PC that will betray the party, just say no. If you really want to try it out, discuss it with the rest of the players first. Yes, this spoils the surprise, but if it keeps everyone friends at the end of the night, that’s worth it.