Typically, being the traitor at Shadows is sort of like playing games where bluffing is involved. Success depends on you managing the table's personalities. Here are the rules I set for myself as traitor and I try to stick to them as much as possible:
Don't lie. This is the number one rule. (Technically speaking the rule should be "don't get caught lying", but I find personally that if I think that's the rule, I spend a lot of time worrying about whether a lie is worth making, whether I can get caught out, etc, etc, which drains away my ability to spend effort on actually playing the game to win as the traitor, so "don't lie" is easier, oddly enough, to follow and win.) The only question you're allowed to lie to (unless it's an official accusation) is speculation that you are the traitor. And you don't want that accusation to ever be made, so...
Refuse to engage in speculation and try, quietly, to defuse it. Don't fall into the trap of parties of speculation, or trying to "defend yourself by accusing someone else". This is a fool's game, because it puts the issue of accusation on the table as legitimate and once that's done, you're as good a target (better, in fact) than anyone else. Do everything you can to get people not to speculate about the traitor's identity or make accusations, which can lead to table management principles...
Be jovial and pleasant, almost carefree. Chit-chat. Encourage others to chit-chat. Be friendly, be funny; ask for the snacks, pass the snacks, enquire after others' desire for snacks, ask if anyone would like more snacks. In other words, as much as possible, try to put the other players in a friendly mental space where they're not thinking too much about the game other than the mechanical "optimal choices" they have to make on their turn, which leads to...
Encourage others to make "the best choice". Get the other players to avoid the conclusion that finding and accusing the right traitor is a wonderfully good thing for them to do; you can do this by focussing on optimal moves whenever possible. Whenever the discussion turns to strategy for an individual player turns, always encourage discussion about evaluating options that don't have to do with the traitor issue. This helps lull your fellow players into a mind-set that the game can be beaten without exposing the traitor, and that exposing the traitor isn't a legitimate strategic option to discuss. While this discussion is going on, try as hard as you can to not offer advice yourself: try to exclusively ask questions, and not answer questions (i.e. "What do you think we should do here?", "Do you think it's better to play a bunch of high cards here?", and so on). This makes you look like you're contributing, but relieves you of the burden of actually contributing to good choice making (and prevents you from being caught lying). Don't overdo it, because this can lead to being suspicious.
Make the others play your hand. This goes hand in hand with the previous point: rather than getting caught out making sub-optimal plays, as much as possible get the others to direct your play. Don't lie, but continually ask the others what they think you should do, how you should play your cards, and so on. You can't break the rules about card-talk, but you can set the others up to spending extra effort to think about playing your hand as well as theirs. Be very careful not to look purposefully dumb (especially if people know you're good at games) and be very careful not to lie (or get caught lying). But spend your thought-effort on figuring out how to damage the group's chances, and not on what the best play is for your own resources: your best play is always to strategically make it harder for the group to beat the game.)
When you have an opportunity to knife invisibly, be merciless. When occasions come up in the game where you can damage the group's chances, with not chance of being caught, do that. Quite often this comes up when you're allowed to examine action cards, defer action cards, re-arrange action cards (as long as you're sure no-one can notice this). Offering bad tactical advice is generally a bad idea (see "don't lie"), but putting the group in the position where they have to choose between a sort of bad thing, and a really bad thing (by quietly taking the good choice off the table) is awesome.
The game presents lots of opportunity for the traitor to submit the group to death by a thousand cuts, so do that every time you can. This makes it far easier for you to...
Stab overtly right at the end only when you're sure it makes you win. If you can make a masterstroke at the end that comes from lying or overt betrayal, do it, if you know the others can't avoid losing the game. This will be a LOT easier if you've submitted the group to the death of a thousand cuts policy: you want, as much as possible, to keep the game tense and tight at the end, because this means that even small choices you make can have correspondingly great effects. You want your masterstroke to be as small, quiet, and deadly as possible because you want to maximize your chances of not getting caught.
