Pharaoh, if Hashem had not hardened his heart, would have had no free will.
Malachim, who do have free will, are unable to “choose” not to believe in Hashem as they “see” Him.
BY: There is a somewhat well known Machlokes between the Maharal and everyone else in the following case:
Reuvain decides he wants to kill Shimon. However, it is not Shimon’s time yet. Can Reuvain use his bechira (IIRC the Maharal) or will outside forces intervene, since it is not Shimon’s time?