Because It Opens The Door to Idolatry. Bitterness devastates our soul and spirit because it is rooted in idolatry. Iniquity in the New Testament means lawlessness. The Bible looks upon a lawless person as an idolater because they insist on worshipping something other than God. Please remember, iniquity is the equivalent of lawlessness and Peter associated lawlessness with bitterness.
Bitterness is a giant emotional step toward lawlessness.
Lawlessness is an expression of idolatry. This was a startling revelation to me. Bitterness in not a stupid, private little game we play in our hearts. It is a risky challenge to God's right to rule. I have two clear choices, I can choose bitterness which is lawlessness and idolatry and risk loosing everything I hold near and dear.