Ok, so I was watching the Chiyo and Sakura fight Sasori, and during all the flashbacks of he's parents I realized that for so many characters it seems like what drives them forwards and make then unique compered to others it's the fact that they lost they parents. I'm just going to share what I remember and think and is interested in hearing you'r opinions on whether it's to many cases of it or if maybe Kishi put a little to much in it. Personally I think it's connected to all the hatred and it being one of the problems with the constant wars, as it's one of Nagato's main reasons for wanting peace.
We got Naruto ofc, which would have been a totally different person of he's parents where alive. Most likely popular(son of Yondaime and not a jinchuriki). H wouldn't have that personality where he is used to fight and take care of himself which is what's make everyone acknowledge him, this is also allot of what pushes him to always train and improve, a deep wish that people will know who he is. He wouldn't become friends with everyone and being able to know what they are feeling and going trough.
Sasori is shown to be happy, kind and loving he's parents and grandma. Then he's parents die's during a mission and he starts doing freaky stuff like making puppets of he's parents and pretending they are real. You can also see him hiding something dark when he is out walking with Chiyo and a little boy falls and hurts himself, but he's parents comfort him and he starts smiling and skip of happy, while Sasori just asks Chiyo to buy him some candy we see him sad and gloomy. And there he's obsession with eternal beauty starts, hence making human puppets which can only be killed/smashed, not age. And making him a dark and twisted person.
We don't get to see any flashbacks that shows Orochimaru loosing he's parents or how he feels about it, but Sarutobi and Jiraya mention some times that it all seemed to have started after he's parents died, tho pointing out that there were signs even before that. And they make it out to be a big reason for him going nuts so to speak.
The first time we see Nagato awaken the Rinnegan is when he's parents are killed. The reason Nagato wants to change the world is because he knows pain and believe that the only way there can be peace is if people know the same pain. The "pain" as he calls it started when he's parent s died and is part of what makes him and Naruto understand each other(Naruto not having parents and loosing Jiraya who was like a father to him).
There are several reasons why Sasuke hates Itachi(later on Konoha because Sasuke sees them to be the reason for it), him killing the Uchiha clan, him torturing Sasuke both mentally and physically, but above all for killing their parents. Whenever there is a flashback with Sasuke it's about either Itachi and/or he's parents, showing that the death of he's mom and dad far outweighs the death of the rest of the clan. He even asks Itachi why the parents in a way that states that the others is not important at all compared to them.
Arguably you one can also count Gaara, even tho he's fathers death haven't even been shown to be of any care to Gaara we know that when he's father ordered him to be killed it was a shock to him, ofc it would be that for everyone which had a father trying to have them killed, but I'm just saying it. And when this happens Gaara made it clear that he didn't want anything ti do with anyone and thus making he's father "dead to me". And this is not just about how ones persons parents death had an impact on them but also people around, like the tip of the iceberg for Gaara going crazy was he's uncle(moms brother) trying to kill him as he had always hated Gaara for causing he's sisters death.
Kakashi could have been mentioned but after he always wanted to be like he's father, but when he died he followed every rule in the ninja book, but changed back when Obito died. So I see no reason counting him.
So if there is anyone else you know of or if you got any comments please share =)