All the cheevos in this game are fairly easy but can get confusing, so I'm going to beak it down for you, stet by step.
1. You need at least 4 people to ges the session starting.
2. Load up Subway, probably the best map for all the online stuff.
3. Run straight to the vault, thake out all the security guards.
4. Gather up all your fellow robbers in the center of the vault.
5. One of the robbers grabs the nearest one from behind (press B), this will give him the "traitor status" then release him (by pressing B again) then KILL'em where they stand:)
6. That's it folks. Simply switch players once the round is over, rinse and repeat until it pops.
The cheevo pops once you've completed the whole set, not the single round.
Ignore the loot, it doesn't matter wether you escape with it or not, just kill your felow robbers.
One more thing, once you get killed by the traitor you respawn as one of the cops. DO NOT move, or do anything once you respawn, let the bots do the killing.
Four people can go from Saint to Scum under 2 hours, and grab a few more achievements on the way if it's done this way.
BTW thanks to Avarat, xGhostFaceKilax and Cloud 995 for helping me with this one.