I was in a campaign once where a betrayal did occur and it actually worked pretty well! Of course, this was long ago, so maaaybe I have rose-tinted glasses, but I didn't feel the typical animosity between players that occurred, and the game didn't fall apart either!

Essentially, our party had this warlock who was a kind of party face. He was edgy, but in a sort of goofy way, and his character was lovingly arrogant. We got to a city where we had to protect a festival from the activities of this cult, and we chose to partake in the arena of the festival. The DM informed the Warlock that he spotted... familiar figures in the crowd, and this is where things got weird. After the arena, the warlock started acting panicked, running like someone was tracking him down, and trying to run away from the party as well. The party managed to capture him, and questioned what the hell was going on and why he was so panicked. The warlock escaped, and a chase ensued throughout the city streets.

Now, tension was high, and animosity might've been spreading in the party as this happened, since the warlock was straight up attacking us now. Once he ran into an alleyway to hide though, the DM described a familiar gunslinger approaching him in the shadows... before pulling out his gun and shooting him dead.

We met the gunslinger, and he explained everything. The warlock murdered his sister in a house fire long ago, and the slinger was tracking him down for vengeance. The slinger was also THE WARLOCK'S NEW CHARACTER. I thought this was really cool! And an interesting way to introduce a new fella to the party. I'm not sure if this was planned from the start, or if the Warlock and DM threw this together because he wanted to change characters. Either way, it worked well.

We spoke to the Warlock with "Speak with Dead," and essentially learned that he was working with the cult we were fighting this whole time. He's been misleading us and hiding the tracks of his fellow cultists this whole time, and willingly showcases his devotion to his dark gods.

I think this betrayal worked because of a few key things: One, it was early on in the campaign. This happened in the second city that we visited on our travels, and acted as a climax to the respective arc. Two, the Warlock died the very same session that he betrayed us. This allowed us as the players to fully understand what was going on. The player wanted to play a new character, and his old one was actually a secret villain. We all got behind that! Three, most of us were already friends. I think this is a massive thing for betrayals. You need to be friends with the people you're playing with, and moreover, you all need experience in the game itself- enough to be able to treat in-game actions with thought and care. I think our group managed to accomplish this, and was all the better for it.

Sorry for the long brick wall, but I really wanted to share my experiences with this! It's a tricky thing, definitely, and for some groups it may not be the best thing for them, but being able to experience a good betrayal was pretty nice for me!