Like Anna in My Sister's Keeper, Lev was born to die. He's a tithe, which means his parents had ten children, with the intention of donating one child (a tenth of their belongings) to the church. In this case, this means they'll have Lev unwound. The religious nature of this gives Lev a cult-like focus. Check it:
This is what I was born for. It's what I've lived my life for. I am chosen. I am blessed. And I am happy. (1.3.68)
Lev has no problem dying, and he doesn't care if he takes anyone with him, either, which is kind of creepy. When Connor and Risa stop him from being unwound, Lev becomes focused on revenge: "He'll make sure they both get exactly what they deserve" (1.6.36). Yikes.
Lev reevaluates his whole life's purpose (or in his case, his death's purpose) after his mentor, Pastor Dan, tells Lev to live. (The two words sound similar, don't they?) Lev realizes that Connor and Risa are his only friends, and he screwed up their lives, so he tries to make things up to them.
Along the way, Lev meets a boy named CyFi and gets his clothes dirty. These events are both significant. CyFi becomes a friend Lev doesn't turn on for a change, and his dirty clothes are quite symbolic.
All his life he'd worn white—a pristine absence of color that defined him—but now there was no comfort in wearing it. (3.21.63)
The absence of color seems to complement Lev's absence of personality, but as he starts to find his identity, his clothes get colored in, too.
Over time, Lev gains some street smarts and some self worth. He cheats a pawnbroker, and upon getting punched when reuniting with Connor, tells him, "Don't you ever hit me again, or you'll regret it" (5.34.75). Unlike Connor and Risa, Lev significantly changes from the beginning of the book to the end.
Lev still doesn't want to live. He goes from being a tithe to being a clapper, a form of suicide bomber. Remember how he said that he wanted to make sure Connor and Risa get exactly what they deserve? Well, Lev keeps his word, except at the end of the book he thinks they deserve to live.
Because Lev has had no fear of dying since birth, it's no surprise he goes the suicide bomber route. He plans on blowing up the Harvest Camp to save Connor and Risa. Unfortunately, Connor is led to be unwound before Lev anticipates, and Lev's partners, Mai and Blaine, blow themselves up, injuring Connor and Risa. This is the moment where Lev realizes it would serve his friends best if he, too, lived. He doesn't blow himself up, and he saves them, which in a way fulfills that savior complex he's had since the beginning.