Haven't posted this theory here yet, so I thought I would!
Fire's Behavior Change:
So Fire, as I'm sure everyone knows, had a big change in character. Some people have criticized saying it was very extreme and came out of nowhere, but I believe it didn't.
When we first meet Fire, he was a whiny cub, who picked on his sister and preferred the reality of the world, versus playing and having fun with his siblings. He believed lionesses were too weak to be Manes, and therefore didn't want to play Mane and No-Mane with his sister. When their father is challenged, he immediately ran, leaving his siblings behind to save himself. Now, before you say that Starmane eventually told the cubs to run, Fire had already run before this after being told to not move. He was a weak, cowardly cub. After the attack, he does go to his sister, and does seem to have a change in heart after the tragedy and begins to support her more.
When we see him as a young adult, he does show more support for his sister, and actively cheers for her during her assessment. He shows heavy disapproval alongside his grandmother with her new name. When she fails, he's the first to try and comfort her, but was exiled before he could.
This is where the pinnacle moment begins. Fire sets off on his own and is trying to survive. Somehow, he meets up with the band of No-Mane's, perhaps they were still small then and gained more members in the weeks after Fire's exile, eventually growing into the group we know. Proud was the last one to join.
This is where my theory begins: Fire is intrigued by Proud, and they soon grow close. Proud began teaching Fire the ways of his old pride, and the strict traditions of Pride Law they follow. The others in the group are less thrilled about Proud's attitude. We see this dynamic when Proudmane bullies Nothing about her name, and Fire tries to excuse his behavior as "just teasing...right buddy" he says, a nervous look on his face. Tangle doesn't let this slide, saying Proudmane doesn't know when to shut up. This is when we get our first glimpse of Proud's influence over Fire. He doesn't really defend Nothing in this moment, the first view on how he truly views his sister.
After Proud, now Proudmane, takes over the Sisters Steppe pride, he tells Nothing,
"None of this was personal, it's just what a No-Mane has to do to survive...when you find Fire, make sure he knows that."
To me, this shows that Proudmane has told Fire this in the past, trying to explain that the only way for him to survive is to become a Mane. This is why the next time we see Fire, he's going after a pride, and already attempted to take it off screen. His mind is already set on survival, this is why he doesn't really hesitate to "kill" Feather when ordered to by Moonstrike, because his position was threatened. He didn't want to go back to the stretch. Fire, as of episode 9-10, was full survival mode. He didn't really care for Nothing, he didn't care for Feather. Proudmane opened up a new world for Fire, and it made his worst qualities come out. He became more selfish, more cowardly, and more cruel.
This brings on another theory: Nothing never told Fire that Proudmane threatened to kill her... only that Proudmane forced her to leave. Did Fire know more than what he let on about Proudmane's intentions?