That we are self contained human beings isn't obvious to me. All of my opinions, thoughts, and actions are a reflection of my surrounding social group.
So if all your friends starts killing would you start as well? I believe you have free will, your imagination, a brain and you can assess for yourself what actions you eant to perform.
I bet if your situation was such that your friends start killing, you would be in the same situation and start killing too. Yes we have free will, very much so. That is why we kill.