To seek vengeance is to hang onto hatred, and to hang onto hatred is to poison yourself — your mind, your body, your soul. To hang onto hatred is to keep a stream of bad-for-you-in-high-amounts hormones and chemicals flooding through your veins.
Adrenaline, cortisol, serotonin, testosterone and many other hormones become elevated when you feel hatred — but so, strangely, does oxytocin. It literally feels good to hate, and (partially) because of that, the emotion is addictive. Once you hang onto hate, elevate those hormones for too long, your body adapts to them, and you no longer feel “normal” unless you are angry or scared or hateful. Prolonged hate literally rewires your brain to make you want to hate more. To make you crave anger or fear — both primary emotional components of the complex feeling — and ultimately change your behavior or disposition to suit.