When Bella is bitten by James (In Twilight 2008 Release Movie), Jasper also comes and rescues her - by killing James - but how is it that Jasper does not react to Bella's blood? Is it because of "heat of battle?" I also think of New Moon, where she only had a paper cut (but he still freaked out).
Stephanie Meyer has established that Jasper is the most recent convert in the Cullen clan. That, plus his history, make him more sensitive to cravings. In the first chapter of New Moon, Bella says,
Jasper smiled, too, but kept his distance...During the days we'd had to spend cooped up together in Phoenix, I'd thought he'd gotten over his aversion to me. But he'd gone back to exactly how he'd acted before--avoiding me as much as possible--the moment he was free from that temporary obligation to protect me.
I think there were a couple of things involved in his different reactions:
- While he, Alice, and Bella were in Phoenix, he was aware that he was going to be in close-quarters with Bella and was probably well-fed and, thus, less affected by Bella's smell. Additionally, he had the support of Alice who seems to be able to "handle" him.
- The heat of battle probably diverted his attention.
- Having been confined together for a few days, he may have become more accustomed to Bella's particular smell, but time may have dulled that. Sort of like you grow accustomed to the smell of your perfume or cologne, but if you don't wear it for awhile you tend to notice it more once you begin wearing it again. Remember that the events at the end of Twilight and those at the beginning of New Moon span a few months (at least the summer months, and it's unknown how much time Jasper has actually spent around Bella in the interim.
- It's possible that Jasper hasn't fed in awhile at the beginning of New Moon. He is accused of waiting too long between feedings in later books (Breaking Dawn perhaps?), so perhaps this is an on-going problem.
- Immediately before Bella cuts her finger in New Moon, Stephanie Meyer writes:
Before he [Edward] could answer, Emmett bounded through the door. "Just in time!" he crowed. He pushed in behind Jasper, who had also drifted closer than usual to get a good look.
So, perhaps Jasper's proximity to Bella when she cuts herself is really the bigger problem. If he had been standing a little further away, he might have been able to leave the room before he lost it. Realize, this is all speculation. I've written a lot of possiblys, probablys, perhaps, maybes, and ifs. Nothing in here from Twilight canon except what I've pulled from the books.