Professional safe-cracker Zed comes to Paris to help a childhood friend, Eric, with a bank heist. In the cab on the way to his hotel room, the cabbie obtains a prostitute for him. He arrives at his hotel room and is soon greeted by the prostitute, Zoe, who also confides that she is studying art, and has a "very boring" day job. After having sex, they talk with each other amiably, then fall asleep. Their reverie is soon interrupted when Eric barges in and brusquely sends Zoe out of the room, so the two men can get on with their business.
Eric takes Zed back to his residence where Zed meets Eric's friends. Eric explains his plans: the following day is Bastille Day and virtually everything is closed except for the bank they plan to rob, which is a holding bank and is open on holidays. Zed forgoes his rest time to spend the night partying with Eric and his friends among some of the less reputable people of Paris in a cavernous jazz club, which Eric refers to as 'the Real Paris'. During the binging, Eric confides to Zed that he has AIDS, which he contracted through IV drug use.
The next day, Zed is awakened by Eric as they prepare to enter the bank. The team dons carnival masks to hide their faces before bursting into the bank. They quickly kill those who do not cooperate as they escort Zed (who has not witnessed the killings) to the safe so he can get to work. Their plans soon start to disintegrate as the police show up and they're faced with the possibility of going to jail for life or having to shoot their way out. Eric throws an explosive into a vault and enters it (mortally wounding a guard in the process - Zed himself shoots the guard as an act of mercy), finding a large supply of gold bars â but the thieves can't leave the bank alive with their fortune. Tensions become even higher when Zed recognizes Zoe (who coincidentally works at the bank) and attempts to protect her, to the fury of Eric, who viciously slashes Zed's cheek with a knife.
A vicious gunfight between the police, Eric, and the rest of the gang beginsâwith Zed caught 'innocently' in the middle. Eric's men are killed by the police as they rush the bank, and Zed and Eric begin to fight each other. The police shoot Eric to death. He falls on Zed, splattering great amounts of blood on him in the process (possibly exposing Zed to his HIV-infected blood). Injured, Zed is led away quickly by Zoe, who covers for him, stating he is a bank customer. They drive away in her car, where Zoe promises Zed that when he gets well she'll show him the 'real' Paris. The film ends with the implication that Zed contracted HIV from Eric and will give it to Zoe, thereby "Killing Zoe."

Question: Why did Zed kill someone?
Answer: As an act of mercy on the mortally wounded guard
Alternative answer: An act of mercy