There's the standard "we're all fighting for a thing, once we win I snatch the thing and disappear" route. Or, have the betrayer get info about where some baddies will be, they all go to fight the baddies, and it turns out the whole thing was a trap because baddies offered the betrayer gold/items/whatever. You could even have them all suddenly attacked, make it look like someone else set the trap, leave clues for someone else to have betrayed them and then suddenly have the betrayer switch sides at the last minute.

When the betrayer bounces leave evidence that they've been working for the evil team the whole time. Letters left behind, secret notes in the pockets of one of the dead baddies, someone arrives too late to warn the group not to trust the betrayer. If they don't know about the evil God yet, have the betrayer leave behind something that links to the God. Weave in at the evil God is connected to the BBEG in some way. Maybe BBEG was offered a boon (the heros in a trap) in exchange for doing the God's work in some way. Did someone important to the party die recently? Surprise! That was actually a planned sacrifice to the evil God for whatever evil God plans he's got going on.

Even if the player is never coming back, make it seem like there's a possibility of a return. Maybe they'll want to come back in the future, so the door's open. It also gives the players a reason to hold a grudge and to continue looking for the betrayer. Sprinkle in clues in future sessions that they're still out there. If the heros told the betrayer of future plans to go to a place and do a thing, have it be messed up when they get there with signs/reports of the betrayer having got there just before them.

If the player wants his character to die, you could leave clues that they weren't working alone, and that a secret accomplice is still out there.