Vindictive

 having or showing a strong or unreasoning desire for revenge:

"the criticism was both vindictive and personalized"
A person described as vindictive is usually someone who holds a grudge and who always tries to get back at people who they think have wronged them in some way. Vindictive people retaliate against others for any insult or perceived slight. Vindictive people do not let things go.
It is not true that suffering ennobles the character; happiness does that sometimes, but suffering, for the most part, makes people petty and vindictive.
having or showing a wish to harm someone because you think that they harmed you; unwilling to forgive
If you say that someone is vindictive, you are critical of them because they deliberately try to upset or cause trouble for someone who they think has done them harm.