In Eclipse, Jasper shares the story of his "upbringing" with Bella. He was transformed into a vampire to fight as a soldier in an army of newborns under the helm of Maria, an older female vampire, out to expand her feeding territory. After many years, the violence and killing got to Jasper. In his search for a different existence as a vampire, he met Alice, who adopted him into the Cullen family. Yet, he still struggles with being "vegetarian."
The impending war against the newborn army brings out his cunning soldier ways: he believes that Bella should be in the clearing to distract the newborn vampires with her scent and, more importantly, he suggests they "cheat" and drink human blood before the battle, because it makes vampires stronger. For all his personal history, though, he shows no empathy for Bree, the young vampire who surrenders, maybe because she reminds him of a past he doesn't want to face.
Jasper also plays a key role in New Moon, so check out his New Moon "Character Analysis" on Shmoop.