I think you have a few options which haven't been thoroughly covered yet:
Try splitting the group up. Get two or three on their own, set them up with a crossbow you found, shoot/stab one and make the other(s) see you. They'll chase you and you can lead them to their allies with cries of "Help, he's gone insane! He killed that other man! Save me!" This has the added bonus of the rest of the group possibly killing the guy chasing you.
Various poisons, traps and magic work wonders. Again, at least the appearance of insanity may be your best bet. The appearance of disease, especially in a medieval setting, could strike fear into the group members. They may drive away or even kill the infected. Try to be surreptious and make sure that no-one suddenly froths at the mouth unless you have an idea of how you can explain it away ("Oh no! Your mushroom soup must have had a bad mushroom in it!").
Stab them in the night, and then 'hide' the dagger near one of the other group members. When the rest of the people wake up, you shouldn't have much trouble 'finding' the proof, or getting someone else to find it for you. This could make the group start to break apart, and a lot of distrustful, armed men might be manipulated into fighting each other, or can serve as an excellent cover for your depredations. This is similar to the next point.
Basically, get a scapegoat. Find someone, either a bystander who is in a position to appear responsible for your attacks, or the member of the group who is most likely to be believed to be a scapegoat by the others and who is least likely to think it's you.
Be a victim. The other people will trust you more if you appear to be suffering as much as they are. If you can, disappear and attack them, then let them find you tied up in the bushed with a cut down your face. Don't blame outright unless you've set up enough proof to make sure you don't look like you're trying to get everyone at each others' throats. Make sure that if the group finds mysterious tracks and follows them, then the guy at the back dies, you were right next to the leader (illusions are great for this if you have a magic item that can duplicate them, as is mind-affecting magic).
Take your time and cover your tracks. Don't rush anything, and be careful to not be suspected. If you work too quickly, then by the time that you've killed half the group, the rest might notice that since the little kid joined them, they've been dropping like flies. Make sure that whatever they think, it isn't going to hurt you.
That said, in some cases, you won't want the targets to have plenty of time to decide what is happening and what to do. Make sure that you always have a good plan, but feel free to put the pressure on them to do something, and quickly, which could be dangerous. For example, if there's a dangerous forest full of monsters, you could try to lure them in without giving them time to gather information on it by injuring one of them and telling them about an old (fictional) healer who might live in the forest, and "who is the only one who can get to him quickly enough to save him, if we hurry!"
There are a lot of things you could try, and I'd suggest using those that look best to you and combining them for a cover that the DM can't auto-fail you on, without playing your whole hand in one go.