I'd go for a bard, or a bard/rogue. Decent combat skills (not great, but not horrible...) good array of mind-affecting magic, and almost all the good stealth/killing skills built-in. Bard 6+ also has an interesting stealth kill: poison food, then use fascinate+suggestion to convince the target to eat it. It's nasty, because at a party with a lot of food, the target's likely to be eating a lot anyway. So it'll be perfectly reasonable, and thus, the save will be at a penalty to the target. And since they are fascinated, they're already at a -4 to perception checks to notice the poison...
how he works:
1) Get close to target
2) Get work at party with target present
3) Get on stage, do his bard schtick
4) Get off stage and let next act move in
5) Get target into his grave via poisons and the occasional heavy-hitting magic.
6) Get paid for music job
7) Get out before anyone notices
8) Get paid for kill
Basic idea is that he has a reason to:
A) Be near his target when the kill goes down, so no guards will suspect his presence of being a bad thing.
B) He's always killing in places where a whole bunch of people are around, throwing off suspicion.
C) He can use his mind-affecting magics more easily, since he doesn't have to travel far to do it, and they're already listening to him.
D) He travels a lot, so no one will mind when he leaves town, particularly when he leaves town before the body is noticed.
E) He's got another job, so when he comes into a town, he won't look odd hanging around casing the kill spot.
F) He's got his weapons in a lute case.
So yeah, basically, his trick is that he's NOT stealthy, and specializes in kills that don't necessarily look like kills. When he needs to leave, he's able to alter self into his target and then walk out, ensuring no one realizes he actually killed the target until well after he's gone and also providing his own means of escape. Also of note: he's capable of disguising himself via magic and skill, so he can disguise himself as one person when he does the job, alter self into another, and if someone uses True Seeing on him, they STILL can't figure out his identity. Nice thing is that he's got so many backups to his magical bag of tricks that he can function reasonably OK without them.
Basically, Bard 8/RogueX works like so: Preload w/ Glibness (+30 to bluff and can't be magically detected as lying.. good as gold for all but the most dedicated of anti-bluff specialists) and Misdirection to a creature or object w/ no aura of magic. hit target with delay poison, then use fascinate+suggestion to get target to poison themselves and leave area, so that they're hidden. Make your way off-stage, ensure your target's death, leave, then D-Door if need be for additional distance between you and the guards. Good things about this: if you do it right, and your target's not so totally crazy or crazy powerful that they're willing to throw out dispels left and right on the off chance that the people involved are buffed to the hilt at a party, you're unlikely to get noticed and your target just seems to have died from a minor illness... The rogue levels would simply help you in sneaking to targets with a bunch of traps leading to them.
EDIT: I've had this idea for a while. Main things I like with this build is that it's workable easily enough just as a pure bard-only build, it's nasty in a way most characters can't touch, and it's pure core. Diplocheese optional, even. (It'll certainly make it more dangerous though.) Another thing I like about the build is that it plays to the heist, assassination, and spy film/book genres, as he reminds me of assassins/thieves like you see in Lucky Number Slevin, Smokin' Aces, The Italian Job, and the Oceans Eleven movies, so it plays back into the bard archetype really well.
