You shouldn't be delivered with hate messages, that would be pathetic on the receiver to do so.
Okay let's get this straight, it was not pointless for Gray to attempt to kill END before he knew about Natsu. His perishing father delivered a message, and as a good son, Gray wanted to fullfill it.
one thing that tends to get overlooked is that Gray is in his corrupted stated is thinking that Natsu lied to him all of this years. He thinks Natsu is aware of being a demon from long time ago etc.When you are corrupted or literally not sane, you tend to believe or think or do things that you normally would not do. That's the case with Gray here.
Gray didn't receive any development after Ul (and hints of Invel) You are confusing development with progression of plot for a character to renact.
. What Gray did against END wasn't development or regression, but in fact staying in the same spot. Meaning Gray wasn't the same Gray against END. Basically he wasn't himself. So I'm not sure why we have to ignite as if it's regular Gray that was attempting to fight END. because that wasn't really Gray at all. I chalk it to an alternative messed up Gray.
It was a simply a corrupted guy who was corrupted with demon slaying magic. So really not fair to pin point that all on Gray Put anyone in Gray's shoes, and they would act the same way he did. Invel practically manipulated Gray around mainly because of his corruption to his magic wearing thin on him, and the fact that Gray just lost another love of his life made him to and things that were out of character.
Basically TLDR, it's unfair to categorize Gray for development in the context against END, because it wasn't him at all that was attacking his friend. It was the magic, corruption and trauma that was making him stay.
Compare Gray's mentality against Ul, and Gray against END, and tell me which one is the gray. Hint: It's the former.
The Gray I know and accustomed from years, would question Natsu on everything he knows.
The Gray I know and accustomed from years, would attempt to protect Natsu from going berserk, or atleast try to demonize the demon in Natsu.
Basically Mashima is making a point, that when you are corrupted with magic, and have shown to be in so much trauma after years of losing so many people in your life that too at young age, you basically become an alternatively different person. Gotta learn from that.
If you watch Supernatural (witnessing Dean and Sam's life), you would perfectly understand Gray even more.
Regardless, people change their minds on characters all the time. So I'm sure he'll be in your tops when he works over overcoming his demon slaying magic corruption and now that he knows Juvia is alive, he'll be acting sane again. Happy smiling and calm Gray is the best Gray btw