Religious extremists, like bin Laden and (in my opinion) Brian Mitchell, are not mentally ill. They have consciously put their trust in false beliefs. They are capable of choosing alternate beliefs. Their actions result from a long line of decisions, not mental limitations beyond their control.
Their enjoyment of fantasy over reality is self-inflicted and enhanced by the choice to use mind-altering drugs. Excusing their behavior as mental illness is an insult to those who suffer from delusions beyond their control or have been brainwashed by others.