Im my opinion it went wrong in season 3. That's were I think most of the fans gave up on the show.
1) The killing off of characters is way to easy. Adam, Arthur, Sam, Maury and Bob (all got killed on season 3). These guys had the potentail to be good to great villians on the show. These writers need to see some sopranos seasons, they never killed off people so quickly. They would wait to the very end to kill off the main character thats going against Tony. And every once in a while they would kill off a main character like Big pussy, Adriana and Chris. Welcome to Heroes you be playing Adam and your Dead! Can I get a line in? NO your dead! Can I put up a fight at least? no you turn to dust, Sorry. lol
2) Nathan Becoming a bad guy. This did not sit well with me at all. Especially after the touching moment Peter and Nathan had at the end of season 1. He should of stayed a good guy that would fight to keep his people a secret and they didn't need to kill him off. He had a great angle in the show being involved in politics.
3) Heroes losing there abilities. Enough said, bad idea. Did superman ever lose his abilites. Only when he became human on purpose.
4) Lets keep Suresh without powers. Can somebody besides Noah be a regular human. Suresh was the voice of reason. He needed to stay that way.
5) I don't know where this came from. But I've heard or might of read that you can't change the future if you go back in time. Because in some way it's going to happen anyway. Let me think for a second. The whole reason why Manhattan was saved is because future Hiro went back in time to deliver a message to Peter. Save the cheerleader save the world. Then present Hiro went into the future and based on what he learned about Ando. He was able to save him by teleporting him back to his office. Time travel is good if its used wisely and if it makes sense to go back and change it. Manhattan blowing up good reason. They can make a whole season on Peter trying to change the past and he keeps coming up short until the very end. That would be cool to watch. Well if I was him i wouldn't like the way things have turned out! So If I was him I would try to save my brother. But lets find away to get Peter's powers back so he can do it.
6) Niki, DL and Micah. They shouldn't have killed off Micahs mon and dad! Do these writers have a heart? And to bring back Niki as Tracey was kind of lame. I liked her as the she hulk.
7) Finish and don't cut short possible interesting storylines. I'm beginning to think these writers have A.D.D.! I'm not going into the plot holes, you all know them.
8) I love Sylar, i think he was a great villian. But after a while I would like to see some type of growth as an evil villian. He's got know purpose besides being hungry for abilities. All the other bad guy's that got killed off had a plan and a purpose. Now I love Sylar and what he does with cutting off the head thing is cool and all. And some you fans like it a little to much which is scare. lol But it was time for him to change. I'm not talking about making him a total good guy. like for instant if some one's a bad guy let him punish him by taking his ability by cutting his head and if someone is good let him use empathy. And I know a lot of people like Sylar the way he is. But him being the same old predictable person wasn't keeping the 10 million viewers that left interested.
9)Now for Elle, she had an awesome ability and good background to get into. The relationship she had with Claire made Claire more interesting. Her Character was growing on me. I felt bad the way her father treated her, that she never went to a carnival, on a date and was tested on since the age of 7. And you have to admit her and Sylar were a good match together. Every other romance on the show did nothing for me. And this show does have a lot of love involved in it. So don't tell me it's not that type of show. They could of been an awesome good/bad board line couple. But for Sylar to kill her made no sense whats so ever. I don't know about you but having abilities must be great. But I take sex over cutting someones head for no reason. And remember his statement from the 1st season. I don't know if I can kill millions. I only kill to get abilities. Now the writers reasoning is that she was only to appear in 13 episodes. I'm starting to wonder if Tim and his writers watch the show. Because then they would of saw the chemistry between them. It would make you think. Hmmmm.... we may have something here. Open your check book and sign her on as regular and get rid of some dead weight! Oh I forgot, Welcome to the cast of Heroes Kristen you being playing the part of Elle and your Dead. lol
There's a lot of other things I could of wrote. But I feel these were the major factors in the lose of viewers. Hey the ratings don't lie. I do love this show and I hope it makes a huge comeback next season.