Lord Buddha said this about your scenario...
"Enmity against enmity can't win"
Karma can be a chain reaction occasionally.If i kill you and you die with anger against me it will create a chain reaction.But if you die with "Metta/Maithree" (a form of unshakable kindness explained in Buddhism) in your mind towards me it will not create a chain reaction.But i will have bad karma.
So your question is, "when it comes to effect me wouldn't someone else come to kill me and wouldn't that person gain bad karma?"
The answer is no, because that is not the only way a karma can come to effect someone.
Imagine i killed you with a gun. So what happened, I sent something through you. Now memorize all the horror movie moments you've had and tell me how many ways are there for a person to die by having something go through his body!
As you can see there is no need for an additional individual for karma to take effect.
Now let's see the same with another individual in the equation.
I have killed you and now i have enough karma to be murdered.And i am no angel,i have many enemies with "small beefs" (quarrel/enmity) against me.
Now as my karma has changed if they try to hurt me there is a huge potential of something going wrong and me dying in the process.They would even be more compelled to kill me.What is important here is that no matter what they still have their heads too.They can choose to not to do anything.But if they try to hurt me i could die even if they have no plans for my death.
I must tell you what lord Buddha said about trying to learn how karma works.
Lord Buddha said only a Lord Buddha can tell the true effect of a karma and no one else is equally accurate in that task.This statement was proved many times by "Arahants" who asked about some people's next lives.