He who kills such a one even knowing him to be a Rakshasa goes to hell. Besides you cannot be killed before the (fixed) time (for your death) comes.
राक्षसं जानमानोऽपि यो हन्यानरकं व्रजेत्। अपक्वस्य च कालेन वधस्तव न विद्यते॥