Karma is not punishment for sins as some teach but it is the grace of God to move us from the animal centred mind to the divine centre of a Light-Being.
Karma is action reaction, outer actions that are followed by inner reactions caused by animal instinctual behaviour which reinforces negativity.
Change your reactions by overcoming urges (Vasanas) to retaliate and you will change your karma.
Meditation, mindfulness, prayer and mantra is the way to overcome the lower karmic vibrations.
Hari OM Tat Sat.