IL law:
(720 ILCS 5/Art. 7 heading) ARTICLE 7. JUSTIFIABLE USE OF FORCE; EXONERATION
(720 ILCS 5/7-1) (from Ch. 38, par. 7-1)
Sec. 7-1. Use of force in defense of person.
(a) A person is justified in the use of force against another when and to the extent that he reasonably believes that such conduct is necessary
to defend himself or another against such other's imminent use of unlawful force. However, he is justified in the use of force which is intended or likely to cause death or great bodily harm only if he reasonably believes that such force is necessary to
prevent imminent death or great bodily harm to himself or another, or the commission of a forcible felony.
720 ILCS 5/ Criminal Code of 2012.
Criminal pulls a gun on the clerk; it is legal for you to use deadly force to protect him/her and/or stop the felony in progress.