How do we purify our bad karma? First, we must understand that our bad karma is the result of this life and previous lives. If we don't recognise what we are doing is bad, then it is difficult to purify. All Buddhas have different qualities. In order to help us, Buddhas manifest different qualities. For example, Vajrasattva is suitable for purification. You can also chant the Medicine Buddha mantra. Theoretically, whatever your practise, if you practise properly, you will also be able to purify, it will also work. Prostrations, prayers to the Three Jewels... all will help us. The main thing is, we need to work on our mental disposition, and not just rely on mantras. If your motivation is not correct, even if you recite many thousands of Vajrasattva mantras, it may not be effective for your purification.

-- Dagyab Rinpoche