Ok. Havent read all the threads, and I have no idea how powerful your character is or how much clout you have or how much favor you curry... but here are a few things to think about.
1. BoVD. The Book of Vile Darkness is your friend. They have tables for magical and non magical drugs whose side effects due to withdrawl will cause paralyzation and death. After you knowingly spike someones drink with it (keeping in mind that its unaffected by dispel magic, remove curse or cure poison, its a drug) Make the arrangement to keep the character supplied by a small servant imp type from your god and keep them weak and locked in a tower. Dispose of at will.
2. Ask your god to leave a Corrupted Creature somewhere in the path of your party. Also from BoVD, its a creature that answers "the calling" of a god or higher power. They return with a moderately powerful magic item (wand/ rod/ etc) and pumped stats ready to sew havok across the countryside for their patron.
3. Rakshasa's are a betrayer's best friend. A rakshasa or a small number of them can be hired or convinced to murder your mates for a major league slice of the loot they carry and usually souls. Let them set up into a local cave, have one come out shape shifted to lure the party in, ambush, and you stay in the back.
4. Is your patron's home plane Icy? Consider giving up a party member to your patron or a high level ranking outsider in a cold, cold place so that they can watch a Shivad (Frostburn p152) Hunt them for sport.
5. Go invasion of the body snatchers on them with Simulacrums.
6. Cast protection from good then protection from evil and gate in a group of angels that were just about to sit down for tea.
7. Sneaky? Ok. Gargantuan Black Dragon that exchanged magical ability for a higher spell resistance. Its standard melee is claw/claw/bite (with chance to swallow) but each claw is adorned with magical ivory colored nails. D4 to see how many hit, each hit drains levels/life. I watched an elf go from jolly to dessicated in one round thanks to those.
8. Keep in mind that outside of the initial sitdown to start doing this your other gaming buds are going to talk to one another. So whatever you do, plan to do it all in one night/session. And lets hope that hey don't read this!!!

Lastly, a one use ring or a contingency type spell to get your character away from danger would be good... but I wouldn't plan on playing that guy again. And *if* this is the first time somethin like this has been attempted in your gaming group, expect to take some grief for it for awhile.