Fear is natural. We fear lions, beasts, fires, insects etc.

This fear is natural and it is not shirk.

What is shirk is when you fear someone or something more than fearing Allah when this person is unable to reach you as in the case when you are at home and you are still afraid to do what he told you not to do though he is not there.

If a person is holding a gun to your head and asks you to insult Allah or he will kill you, you are permitted to do that because you were forced. Allah will not hold you accountable for that as long as your heart is filled with Iman.