It can't be corrected. I thought it was pretty much a sealed deal that this is the last season.
However, as much as I loathed most of seasons 2 and 3, I'm generally enjoying this one, though this episode irked me.
I found the way Nathan tried to commit suicide was pretty lame. The least they could have done is make it be less pathetic. "I give up. Here, let me not only kill myself, but help the guy who killed me."
It wouldn't have been hard. Okay, so you want Adrian Pasdar off the show, and Sylar still around, but you want Peter to seem like a bad-ass. Okay, easy. You let Nathan resurface for a bit, give his talk about being tired, about how he wants Peter to kill him before Sylar takes over again. He yells at Peter to kill him. Peter, having beaten the crap out of Sylar and performed some flagrantly Christian imagery with a nailgun, balks at the idea of murdering his brother.
Emotions get heated, Nathan yells that he needs to die, Peter refuses, they struggle, and finally Nathan turns his back on his brother and falls to his knees. Peter stands over him, telling his brother he's strong enough to fight off Sylar. The camera angle shows Nathan's face, which shifts momentarily to a slyly smiling Sylar, then shifts back to Nathan, still with the sly smile.
"You're right," Sylar says, pretending to be Nathan, "I am strong."
And then he telekinetically hurls Peter through the windows beside the balcony, knocking him indoors. He stalks after Peter, who is too stunned to concentrate on the Haitian's power. Sylar, still disguised as Nathan, starts knocking the crap out of Peter, beats him with a handy pipe that's been banged loose, and he's about to slice off Peter's head when bang! Gunshot.
We turn to show Ma Patrelli, gun in hand. Sylar staggers back, and she pumps a few more bullets into him. Adrian Pasdar's last line on the show is a shocked, "Ma?" as he looks to the mother who killed him. He slumps to the floor as Nathan, and then his face vanishes, leaving him just Sylar.
"What a family," Sylar groans as he starts to regenerate and stand back up.
We snap back to Peter, who concentrates, and Sylar gurgles in pain. He looks to his wounds, which are gushing blood. He glares at Peter, growls, "No," then starts to shamble away, kind of awesomely since he's just been shot with a half dozen bullets. Peter stands to chase after him, but Sylar manages to fling himself off the roof, out of Pete's range, and then he flies away.
End on Peter and his mother, standing at the edge of the balcony, looking up into the sky with despair.
That is how I would have done it.