The whole 'Ellie mad cuz Joel dead, now she kill' story never should've happened at all. It's out of character, it clashes with the grounded and pragmatic set-up of this universe, and it's immediately clear that things are going to end badly for her anyway. It's a story set-up that destroys a character with an extremely minor chance at even revealing anything interesting about that character.Ellie is gay btw. That's her character development really.
But in seriousness, Ellie doesn't really NEED character development because she had that in the first game so the player pretty much knows who she is. The problem is that even at the start of the game Ellie isn't the person we know and love. Even if they started the game with Ellie pissed at Joel for the first game, it still wouldn't change who she is or how she acts with everyone else. Yet she's a very different person pretty much from the first moment of the game.
The shame there is that there could have been a very interesting character study on Ellie if they had made her consistent with who she was in the first game. In fact using all her flashback scenes very early and close together could have shown the play in the strain Joel put on their relationship because of his lies about the Firefly incident. But the scene where Ellie goes to forgive him NEEDED to happen before Joel dies full stop.
Then Joel could still die and Ellie could go into this decent. Even bring Dina along to back her up, but slowly get horrified by Ellie as her thirst for revenge gets worse and worse.
The fundamental problem with TLOU2 is that they tried to play both sides and you end up with a story nobody likes. They needed to focus on one story and tell it better, than trying to double dip, because you end up with characters that make no sense and a lot of development (if it even exists) has to be rushed for time.
No matter how much you try to fix the story as is, the core of the story is so hackneyed and out-of-character that it really makes very little difference in the end.
Honestly, I would've been fine with Ellie going down a dark path and coming apart at the seems, but it should've been due to her finding out the truth, NOT because Joel got killed. Because I'm going say something probably controversial (to fans anyway), but Joel is really not that important to Ellie. I mean, he's important to her, but to act like he's the most important figure to have ever existed in her life, whom she would sacrifice the life she has and her friends and loved ones safety to avenge his death, is really overestimating him. It makes Ellie come across as this naive and sheltered daddy's girl whose never known anyone but Joel, and we know that's not the kind of character she is.
And it's not just Ellie, because the game itself treats Joel with way more reverence than he actually has as a person in this world. I mean, after his death his house is covered in flowers (which they somehow got in the middle of bloody winter) like he was somekind of royalty. Joel is a terrific character, but within this universe he's just another guy, and the game just forgets that and instead infuses his importance with the importance that he has to the TLoU fanbase.
Ellie ends up being a shallow cypher for the audience's rage at Joel's murder, rather than her own character. It's this that really pisses me off about TLoU2.