BG Chap 16 - The demons killed by Kṛṣṇa personally
The placing of the asuras, the demons, in the lowest status of life is simply another feature of the Lord’s mercy. Sometimes the asuras are killed by the Supreme Lord, and this killing is also good for them, for in Vedic literature we find that anyone who is killed by the Supreme Lord becomes liberated. There are instances in history of many asuras-Rāvaṇa, Kaṁsa, Hiraṇyakaśipu—to whom the Lord appeared in various incarnations just to kill them. Therefore God's mercy is shown to the asuras if they are fortunate enough to be killed by Him.
Although persons who become demons by curse externally show hatred to the forms of Viṣṇu like Vāmana, Nṛsiṁha and Varāha, internally they are followers of the Vedic path, and consider Him to be the Supreme Lord, the controller of time, beyond material vision. Being defeated by Vāmana and other incarnations, they eventually give up the bodies of asuras. And those killed by Kṛṣṇa like Kaṁsa attain liberation, which is only attained by the perfection of bhakti and jñāna. Kṛṣṇa gives such sinful asuras liberation because they do not oppose scriptures.