Q:

Why did Peter Petrelli need Nathan to save NYC? [ spoilers]

At the end of season 1 of Heroes, disaster is narrowly avoided when Peter Petrelli, who cannot control his acquired power of radioactivity and is about to go nuclear and kill most of the population of New York City, is lifted up bodily by his brother Nathan and flown off into the sky to explode there. 
Now, we know that Peter absorbs other people’s abilities, and that flying was the first one he absorbed, from Nathan. 
So why was Nathan needed at all here?  Peter could fly as well.  Why didn’t he just fly off into the thermosphere by himself and explode to his heart’s content without killing his brother in the process? 

A:

I believe it had been mentioned previously that, having absorbed so many different powers, Peter was having problems controlling all of them at once.  The problem seem to get particularly bad once he absorbed Ted's nuclear radiation ability.  Note that Ted's power, was shown several times to become a "runaway reaction"; it happened to Peter twice and Ted at least once (in the Bennett house).  Unlike most of the other powers Peter had, it took major effort just to stop that nuclear power from growing worse.  Peter wasn't able to control that power and use his other powers at the same time. 
Earlier in that same episode, we see him nearly go nuclear before passing out; that time, as well, he was struggling just to keep himself from exploding, and it overwhelmed him so badly he was rendered unconscious. 
Remember, by that point he had knowingly absorbed at least two other powers that could have saved him: not only Nathan's flying ability, but also Hiro's time and space manipulation. ( He also may have been close enough to get DL's phasing ability, but he wouldn't have known it.)  But focusing any of his effort on using those abilities would have risked letting the nuclear explosion slip out prematurely.