in the AU episode following a timeline in which peter *did* blow up new york due to not being able to control one of his powers, i would argue his character is approaching more of an anti-hero even though he doesn't specifically do anything anti-hero-ish. it is more his attitude since he already believes he has so much blood on his hands and he sees himself as entirely responsible. this makes him in a way more violent--he has killed before so he's more willing probably to kill for the "greater good." he no longer sees himself as a good person.
also in this AU future, peter and his brother nathan (the most important person to peter) are estranged.* peter i think is always darker when he can't believe in nathan who he really needs to have as an example to look up to. basically when nathan goes dark, peter tends to lose faith. (*it's actually nathan is dead and sylar is impersonating him, but peter doesn't know that until later.)
peter is deceived by adam who wants to unleash a deadly plague upon the world. he trusts adam because of adam's self-presentation and apparent ethics (adam seems like someone trying to do the right thing and claims he's trying to prevent the plague). but the big reason he trusts adam is because adam healed nathan. at the end of season 1 nathan is badly burned and injured because he had to fly peter away from new york so he wouldn't blow it up with his loss of control over his powers. that adam would heal and save nathan cements peter's belief in who adam is and that he can trust him. also, in a way adam heals peter as well by doing this. at the end of the season peter almost finishes helping adam unleash the plague until nathan shows up and helps him realize that he can't trust adam. also in the season finale peter was okay (i think i recall) with injuring or killing guards in the building the deadly virus was kept in, in order to get to it. he had seen the future in which the plague is unleashed and lost someone else important to him there and is willing to go to any lengths to prevent it from happening. with a lot of power and when stakes are high, peter tends to become an extremist and this is just a small taste of it.
peter from the future who *is* an anti-hero goes back in time and attempts to murder nathan to stop him from revealing to the world that there are people with special powers in a political speech. i thought this was so terribly out of character. it's like they forgot that peter is primarily motivated by his relationships--as i saw him, if you push him to it he would probably rather a great many people die than lose his brother, so to actually try to kill nathan was bizarre to me. in killing nathan, 'future peter' is trying to save the world of course. apparently thanks to nathan's speech, abilities later end up being sold to consumers. just a shot and you can have magical powers too--and you won't know which power you'll get. with all this power everywhere (some abilities are both extremely deadly and hard to control) those with the visionary ability all see a future in which the earth will explode. but future peter's "solution" was, imo, poorly reasoned (which *is* in character for peter
) to the extent it seemed completely unjustified. the only way to prevent this future is by killing nathan before he can spill the beans about abilities? really?
it possibly was a little more complex in that in the future that 'future peter' came from, nathan is trying to kill him, so their relationship has been broken. 'future nathan' considers 'future peter' a terrorist and although i don't see everything 'future peter' does, i imagine he might even be hunting people with the 'exploding bomb' ability or other powers that could be similarly devastating. i also don't know if this was future peter's first attempt (to save the world). it's possible he tried non lethal methods before but found he couldn't change the future.
it's also true that nathan is himself an extremist and once he gets some notion fixed in his head he's relentless and single-minded in pursuing his objectives. nathan is kind of a faltering force to reckoned with. he's difficult to stop, but he's also kind of a tool and eventually he can be made to falter in his resolve, especially if he sees his course is getting out of control and leading to disaster (although this is another reason why i don't see why killing him would even be necessary, though it would take some manipulation probably which is maybe less straight forward than peter's approach). nathan believes the ends justify the means, but he needs the ends to be good usually. (although 'future nathan' from the other timeline really did seem darker, as though maybe he's not even trying to do the right thing anymore...)
to make matters worse, 'future peter' brings 'regular peter' to his future to show him what will happen. but 'regular peter' ends up acquiring sylar's nasty ability there and later kills 'future nathan' with it. apparently sylar's ability "to understand how things work" comes with the uncontrollable urge to cut open someone's skull in order to "understand" (that wasn't simply the way sylar chose to use it, as i would have preferred). what was interesting about this (other than that this was actually peter killing his brother
) was that he was trying to understand 'future nathan's intentions. this was not the nathan peter idolizes and looks up to and peter couldn't understand him. the show never tells us, but i wonder if this made him doubt nathan later (that maybe he's been blinded to who nathan actually is by rose colored glasses). as far as the show on the whole goes it's interesting because nathan is often someone's puppet (even his form remains for puppeteering after he dies) and it raises the question of whether peter does the same thing really: "i need you to be who i imagine you to be." often everyone's "image" of nathan is more important than nathan; and nathan himself sometimes seems like only an image (enneagram 3 probs suck).
honestly, i see 'future peter' as more of an ESI; but his mini storyline demonstrates kind of how extreme the character can get. (of course i'm sure the writers were busy orgasming over it, but anyway.
returning to the main season 3 timeline, the future in which the earth explodes starts appearing again, and nathan is still on some dangerous path (albeit a slightly different one). even though peter knows the inevitable future he fails to talk nathan out of his path and ends up threatening his life (pointing a gun at him). nathan feels deeply wounded and betrayed by this (not to mention by the revelation that his brother from the future tried to murder him earlier, which he only knows because 'future peter' the fuck up confessed it to him lol). this creates the rift between the brothers for the remainder of season 3. i think really this is what the creators of the show wanted, to pit them against each other for the sake of drama. it didn't need to make sense.
peter tortures sylar trying to force him to take on (keep) the form of nathan (nathan was killed by sylar at the end of season 3 and matt parkman manipulated sylar into believing he actually *was* nathan... it's complicated). this is because peter just wants nathan back and will even accept an illusion because it hurts too much to accept that nathan is gone. i could actually see an EII doing this. by this point peter's character has been through a lot of shit and it cost him the person he loves the most. his sanity has been a bit stretched, and an illusion delays the pain of basically accepting the end of the world (a world without nathan). he also hates sylar for killing nathan and wants revenge.