The Lookout

While driving his car with the lights off, high school sports star Chris Pratt crashes into a combine stalled on the road. Two occupants of the car are killed while Chris and his girlfriend Kelly survive. However, the crash leaves Chris with lasting mental impairments, including anterograde amnesia, along with some anger management issues.
Four years later, he's in classes to learn new skills, including the simple sequencing of daily tasks to compensate for his inability to remember, and keeps notes to himself in a small notebook. Challenged by a tough case manager to build a life despite his injuries, he is emotionally supported by his roommate, a blind man named Lewis, but receives only financial support from his wealthy family. Chris works nights, cleaning a small-town bank. Aside from Lewis his only friend is Ted, a seemingly clumsy Sheriff's Deputy who checks in on Chris regularly. Chris repeatedly tries to convince the bank's manager, Mr Tuttle, to allow him to apply for a teller job, to no avail. Chris soon comes under the scrutiny of a gang planning to rob the bank. Their leader, Gary, who knew Chris from high school and resented his wealth and popularity as a hockey star before his accident, befriends him and uses a young woman, Luvlee Lemons, to seduce him. Taunted by the gang about the limitations of his life since the accident, Chris initially goes along with the scheme. His frustrations trickle down into confrontations with his friends, Lewis and Ted.
When the gang arrives the night of the robbery, Chris tells them he has changed his mind. But they tell him it's too late and force him to empty the vault at gunpoint. His friend Ted, the deputy, stumbles into the robbery while delivering doughnuts to Chris, and triggers a shootout. The deputy and two of the gang members, Marty and Cork, are killed. Meanwhile Chris escapes in the getaway car, and when he realizes he's got the money they stole, he returns compulsively to the site of his accident, where he buries the money roadside. Gary is seriously wounded and gets away with the other bank robber, Bone. When Chris returns to his apartment he sees the lights on and realizes something is wrong, and when he calls discovers Gary and Bone have taken Lewis hostage to get the money back. Chris, using his new sequencing skills, hatches a plan to stay alive and save his friend. But the robbers catch him napping at the place they arranged to meet, and they force him to take them to the site of the buried cash.
While Chris digs in the snow to retrieve the money, Gary's condition is rapidly deteriorating. Chris gives one of two bags to Bone, who is preparing to execute Lewis, but Chris uses the shotgun he stashed in the other bag to shoot and kill Bone before he can react. Gary collapses and dies. Chris returns the money and turns himself in, but the investigation by the FBI concludes that he was not responsible due to his medical state - and because the robbers failed to disconnect the video surveillance in the bank, allowing the FBI to see the gang forcing Chris to act at gunpoint.
In the aftermath, Chris and Lewis reconcile, and open a restaurant together with a loan from the bank. Chris hopes Kelly will forgive him for the loss of her leg in the accident, and that one day he will find the courage to talk to her again.

Question: Why does Chris kill Bone?
Answer: Bone is about to kill Lewis