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.
Vindictive people have a disposition to retaliate for wrongs, real or imagined.
Their impulse is to try and inflict suffering or punishment as retribution for evil or an injury, real or imagined. They are ugly people to be around because even their countenance testifies against them. Inordinate ambition and inordinate competition are also mental attitude sins. There's nothing wrong with ambition or competition but when a person is obsessed with them, there's everything wrong. They're always competing and comparing with others.