First and foremost how do your other players feel about the entire situation? I have no issues with having a mix of evil and otherwise aligned PCs all in the same group in my games, so that's not my issue here, but the point of the game is to have an enjoyable shared experience. If one player is consistently working against the rest of the group they had also better be okay with that or it'll wear thin rather quickly and you're going to have more problems on your hands.
Next, I want to make sure I understand this correctly: one PC was evil and working with the primary antagonist against the rest of the group, actively trying to kill them all. This PC made their move, completely botched it, and is now going to die for his betrayal. Now, and here's where I lose you a little bit, the player wants to make another evil PC to try and do the whole thing all over again? That is, make a new PC who is working with the primary antagonist for the purposes of killing the rest of the party? In short, he wants a do-over?
See, this is the sort of thing I'd have an issue with... assuming I'm interpreting the situation correctly. I'm fine with parties that are any mixture of alignments, I don't generally say no to that kind of thing until it becomes an issue that's causing my players to have less fun because of it. This strikes me as different though, because in this instance you have a player who is just consistently trying to work against the group. Doing it the once can add a lot of fun twists to the game and it's not a bad idea in the least. Doing it again and again? It just "feels" wrong. Your player had a good idea that hopefully added something meaningful to the game for everyone, but whether it was poor planning or the dice were against them the plan failed and his character got caught. Simply trying to repeat the process again cheapens the whole thing and simply starts to feel like this player has infinite chances to try and kill all the other characters in the game. That doesn't sound fun to me.
Again, I could be misinterpreting the whole situation, but that's how I read it. Do whatever will be the most fun for you and your players, as a whole, but I probably wouldn't allow this particular player to run another evil PC intent on acting against the party because he already had his chance and now it's maybe time to try something new.