Hi,
I think you gave the answer yourself :) That's the 3rd force: There's good, there's bad, and there's the specific situation which determines which is which. In one case violence will be STS, in another it will be STO. And the same for non-violence. I think it depends on your motives. Sometimes violence is right: to protect an innocent for example. You act out of "righteous anger".
My 2 cents anyway.
NB : your question makes me think of a
video which has been spreading on the "French" internet for a few days. An agression filmed by the Big Brother' camera in a bus (very efficient isn't it). We see a man getting attacked by 4 guys who steal his purse. He tries to get it back but then gets beaten up for several minutes. A couple of people try to protest and gets beaten as well. Now, the bus driver does nothing during all the time it happens. And most people in the bus don't react either. These guys didn't seem to be armed or whatever. In that case, I really don't think it's STO to just stay there and don't move a finger. I don't say I'm "better" than these other people, I probably would have been too scared, like them, to do anything. Still, imagine if all the people in the bus reacted to protect the attacked guy. Same on a big level : if all the people united NOT to fight the psychopaths but to protect eachother from them.
Sorry for the digression.