So Lord Krsna appeared primarily to deliver Devaki, rather than kill Kamsa

Bhagavad-Gita

TRANSLATION

In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium.

PURPORT

According to Bhagavad-gita, a sadhu (holy man) is a man in Krsna consciousness. A person may appear to be irreligious, but if he has the qualifications of Krsna consciousness wholly and fully, he is to be understood to be a sadhu. And duskrtam applies to one who doesn't care for Krsna consciousness. Such miscreants, or duskrtam, are described as foolish and the lowest of mankind, even though they may be decorated with mundane education; whereas another person, who is one hundred percent engaged in Krsna consciousness, is accepted as sadhu, even though such a person may neither be learned nor well cultured. As far as the atheistic are concerned, it is not necessary for the Supreme Lord to appear as He is to destroy them, as He did with the demons Ravana and Kamsa. The Lord has many agents who are quite competent to vanquish demons. But the Lord especially descends to appease His unalloyed devotees, who are always harassed by the demoniac. The demon harasses the devotee, even though the latter may happen to be his kin. Although Prahlada Maharaja was the son of Hiranyakasipu, he was nonetheless persecuted by his father; although Devaki, the mother of Krsna, was the sister of Kamsa, she and her husband Vasudeva were persecuted only because Krsna was to be born of them. So Lord Krsna appeared primarily to deliver Devaki, rather than kill Kamsa, but both were performed simultaneously. Therefore it is said here that to deliver the devotee and vanquish the demon miscreants, the Lord appears in different incarnations.

(Bhagavad-gita 4.8.)