Blind Al: Can you speak up? It’s hard to hear you with that pity dick in your mouth.
Deadpool: Pump the hate brakes, Thanos.
Deadpool: So you're an alien, I guess. How does that help us?
Shatterstar: I'm basically better than you at everything.
Deadpool: Just once, I'm gonna find a planet of people that are worse than me at everything. A bunch of functional idiots. I'm gonna go there, and I'm gonna be their Superman.
Weasel: Isn't that Canada?
Deadpool: You shut your goddamn trash mouth.
Deadpool: Give me your best shot, One-Eyed Willie!
Deadpool: So dark. You sure you're not from the DC universe?
Juggernaut: I'm gonna melt you down and make a cock ring.
Deadpool: Well, that's just lazy writing.
Ryan Reynolds: [Looking at Green Lantern script] You're in the big leagues now, kid. [Shot in the head by Deadpool].
Weasel: It’s time get back on LinkedIn.
Answer: His change of heart came from Deadpool's sacrifice. In the second timeline, Cable saves Deadpool, but Deadpool had no way of knowing. Firefist still has a change of heart because Deadpool was willing to sacrifice himself, even though he was ultimately saved by someone else.
Now, I can accept that in theory, except that Russell repeatedly saw Deadpool putting his ass on the line to rescue Russell. I mean, Russell knew from the very beginning that Deadpool could have killed him (but chose not to) and took some severe ass-beatings on Russell's behalf. Russell was really, really hard-boiled, and I'm not seeing that Deadpool almost getting killed as enough impetus to change Russell's heart. It seems (to me, anyway) it was Deadpool's actual death that changed Russell, such that a mere deflected bullet would not have the same effect.
Charles Austin Miller
Deadpool often mentioned "lazy writing" and Russell having such a change of heart might be an example of it.
Erik M.
Deadpool saving Russell in the film is what made Russell think that they were friends. When Deadpool tells Russell that they aren't friends, he remains hostile toward Deadpool, not believing him when he later admits to caring for Russell. At this point Russell is too far gone and will kill. However, it's only when Deadpool takes a bullet for Russell, fully intending to die in both timelines, that Russell sees that Deadpool really does care about him, and would have died to save him.