Just like how the mind has its negatives, the heart does, too.

In general, the heart is very forgiving, but sometimes the heart can't handle it anymore, and goes a little crazy. The heart that was once, very forgiving, but now, filled with hatred. 

The fire inside it burns, and it feels as if it will never be put out until, it gets what it desires, and that is, revenge.

It drags your mind into its craziness, and you are unable to think rationally, the mind has become your heart's slave, obeying it's every command, as it comes up with devious plans.

Sometimes, the fire within you will extinguish, and it would just be a short-lived hatred, but it doesn't always happen, so be careful with what you do now, and try not to let your feelings get the best of you.