- In the comics, it takes being fatally shot to kick-start Wade Wilson's Super-Cancer into his Healing Factor. In the movie, it takes Weapon XI being fatally injured by being cut and probably having his torso smashed to kick-start his Deadpool-ness. It may have been an Author's Saving Throw, but it works.
- That, and Wade Wilson's stint as Weapon XI? What sort of horrifying experimentation and mental conditioning do you think he went through? It's the perfect set-up for the famous Deadpool insanity! And it's even followed up on in one of the endings, when Deadpool's head looks at the camera.
- How did Logan grow up to be such a decent person with a sibling as violent and feral as Victor? Well, just compare the way they were raised before they decided to run away from home. Logan was raised by his legal father, John Howlett, who was a kind and loving man who taught Logan the value of kindness and mercy while Victor was raised by both him and Logan's biological father, Thomas Logan, who was a dipsomaniacal and unkind man, who would probably beat Victor out of spite, unwittingly teaching him the knowledge of pain and hatred, to the point of possible trauma. So, as they entered the battlefield, Logan would keep the good ethics taught to him by his legal father to control his own anger while Victor would succumb to his hatred and unleash his sociopathic urges on both his enemies and allies, turning him into a monster far worse than his father. In short, the way they were raised in their childhood is what shaped them to who they are now.
- The previous films established that head injuries (blunt impacts or gunshots) will knock Logan out, while Logan easily shrugs off a headshot from Agent Zero. Considering Logan establishes that his healing factor was deteriorating due to the Adamantium, it makes sense for Logan to be immune to such injuries during a period where he hasn't been bonded to the metal for too long yet.
Fridge / X-Men Origins: Wolverine