That is nothing more than an argument from adverse consequences, a failure of reasoning. In light of the fact that the bulk properties of human behavior are thoroughly deterministic on a mathematical level (even though we cannot always make precise predictions ourselves), it is incumbent on others show that a mass murder is not on a deterministic path.
Choice is an illusion, an assertion supportable by the measurable determinism of human choices in practice. A system capable of supporting genuine free will would have very different behavioral characteristics.
Did the murderer have a choice? We as a civilized society evidently believe that he did or else our putting him to death is truly a failure of reason.
Then again, what would be the purpose of reason at all - or morals - or laws - if all decisions are the involuntary consequences of the physical brain?
Getting back to observation though - many (if not most of us) observe that we and others exercise free will. And not only creatures with brains, but creatures without brains also make choices.