In ep 4 you find out Christa is pregnant and Omid's leg is infected. He's sick and getting weaker.
Kenny does what he does and is about to kill Omid because he thinks he's going to die and turn.
Christa is pissed and aims a gun at Kenny with the intent to kill and Kenny doesn't see her.
If you try to warn Kenny he'll look at Christa and get shot. Killing Christa is the only way to prevent his death.
You have to take out Christa or let Kenny die. What do you do?