Isaac learned that you cant change the future, thats why he didnt even bother fighting Sylar
What was more interesting is when Peter told Nathan that he cant really die, and when Nathan asked him if after he explodes, could he survive... Peter said "probably, but do you know how many other people would die?.... 0.7%" - The look on Nathan's face made you think that he would go along with Linderman's plan as long as Peter lives
Next episodes looks really good...
i think isaac sacrificed himself to create a path to sylar's defeat. after all, he's the future boy. maybe he knew he had to die for sylar to be defeated. maybe if he lived things wouldn't go right. maybe that's why he said he could be a hero if he sacrificed himself hiro made it pretty clear that you can't change the past, only shape the future. i guess after this season ends we'll have a few more clues regarding the shows space/time mythology. i'm just taking some guesses here.