Every believer who does not have a personal relationship with God eventually will become a walking timebomb ready to explode. When we allow bitterness to cause us to explode, either inwardly or overtly, we hurt our own life. We call this self-induced misery. Harboring bitterness is a very serious spiritual problem. A problem which is not correctable apart from personal love for God and impersonal unconditional love for all mankind.
Bitterness produces a complaining attitude, and no one likes to be around a complainer.
People who habitually complain are bitter people and is simply an outward effect of an inward sin. Bitterness makes a person miserable. Vindictiveness is where revenge sets in. Getting even, vindictive people are obsessed with seeking revenge. Vindictiveness is marked by or resulting from a desire to hurt and be spiteful.