We cannot hurt Allah.
As far as shirk being worst sin: Imam Ja’far as-Sadiq (a.s.) says:
“The greatest of the Greater Sins is to attribute partners to Allah.”
The following verse of the Holy Qur’an asserts:
“Surely Allah does not forgive that anything should be associated with him, and forgives what is besides that to whomsoever He pleases, and whoever associates anything with Allah, he devises indeed a great sin.” (Surah an-Nisā’ 4:48)
It means that whoever dies as a polytheist, there is no chance of his salvation. Although Allah (S.w.T.) may forgive any of those who have committed any other sin. Allah (S.w.T.) says:
“Surely whoever associates (others) with Allah, then Allah has forbidden to him the garden, and his abode is the fire; and there shall be no helpers for the unjust.” (Surah al-Mā’ida 5:72)
Also the Holy Qur’an states:
“Do not associate aught with Allah; most surely polytheism is a grievous inequity.” (Surah Luqmān 31:13).
“And whoever associates anything with Allah, he devises indeed a great sin.” (Surah an-Nisā’ 4:48)