Bitterness is having feelings of hatred and resentment in your heart. These emotions are sinful and it allows Satan to come in to kill and destroy what is good and may cause evil.
Eph 4:31,32 004:031 Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: 004:032 And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you. Matthew 6:14,15 006:014 For if ye forgive men their trespasses, your heavenly Father will also forgive you: 006:015 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses. Romans 12:14-21 012:014 Bless them which persecute you: bless, and curse not. 012:015 Rejoice with them that do rejoice, and weep with them that weep. 012:016 Be of the same mind one toward another. Mind not high things, but condescend to men of low estate. Be not wise in your own conceits. 012:017 Recompense to no man evil for evil. Provide things honest in the sight of all men. 012:018 If it be possible, as much as lieth in you, live peaceably with all men. 012:019 Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord. 012:020 Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. 012:021 Be not overcome of evil, but overcome evil with good. Hebrews 12:14,15 012:014 Follow peace with all men, and holiness, without which no man shall see the Lord: 012:015 Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled; Luke 23:34 Then said Jesus, Father, forgive them; for they know not what they do. And they parted his raiment, and cast lots.