Building good karma means that we should have good karma with all sentient beings, not bad karma. The purpose of studying Buddhism is not only to save oneself but also to save others. To save oneself without saving others is called a Han. This is the realm of Hinayana. The highest one can become an Arhat. And Mahayana must save others after oneself saves them, only in this way can they become Bodhisattvas and Buddhas. If you want to save him, you must have a predestined relationship with all sentient beings. Otherwise, even if you succeed, you will not be able to save sentient beings who have no predestined relationship. And karma can be divided into good karma and bad karma, so we should make more good karma and avoid bad karma.