And though trzaska has a point, and I've read the books and so know that Littlefinger will betray Ned, even from the show, just by looking at Littlefinger and seeing his behavior and characteristics, I thought it fairly obvious that this is a person that might help Ned, might betray Ned, but would ultimately do either for his own reasons and to benefit himself.

Him getting his hands dirty by being the one to put a dagger to Ned's throat is another ridiculous way of hammering in the point that he's an unscrupulous bastard, for those viewers that still missed that point, and it goes against his character. Littlefinger isn't a warrior, and isn't the type to get his hands dirty in such a way himself. What's the point of hiring several thousand of the city watch if you still have to do their work for them?