I liked the lower strings part, but I have to agree that it doesn't sound very dangerous. Maybe some dissonant or augmented chords could add some tension. I would also agree with the dynamics idea, it would help the tension rise a little bit, especially because of the repetition. I don't know about the flute, it makes it even less menecing. You could try some chimes or glockenspiel to replace it if you were going for a contrast between innocence and danger (especially with the glockenspiel.) That may sound cliche, but it can sound creepy if done right. I like the percussion idea, but it doesn't seem to fit. Maybe if you use that instrument earlier in the piece the ear will be more prepared and excepting of it. I think you have a good start! Keep experimenting and see what happens!
