If you want to think the best of the angels who bred with women, you could surmise that they had seen the fallen nature of humans, how that they were physically degenerating since they came under God's curse and started dying, and that the angels may have thought that if they took on perfect human bodies and bred with humans that they could restore the human race back to perfection. However, that didn't work, and they became lustful and sinful (Jude 1:7, 2 Peter 2:4).
If you don't believe that the angels took on human form (as men) and mated with women, forcibly taking "any that they wanted for themselves as wives" (Genesis 6:2, WEB), then you will not fully appreciate why God commanded the Israelites to kill all people (men, women and children) that were living in certain cities in the promised land, Deuteronomy 20:16-17 (WEB):
It was because these people's DNA had been corrupted by the hybrid race that the angels had produced - the Nephilim. Satan was trying to thwart God's plan to provide a Messiah to redeem all mankind, so God told the Israelites to destroy those hybrid beings (their human DNA being mingled with non-human DNA). The Messiah's sacrifice of his life in Adam's place could only redeem those people who are descended from Adam and Eve. Anybody who had been born as a result of cross-breeding with angels would not be redeemed.
If you don't believe that the angels took on human form (as men) and mated with women, forcibly taking "any that they wanted for themselves as wives" (Genesis 6:2, WEB), then you will not fully appreciate why God commanded the Israelites to kill all people (men, women and children) that were living in certain cities in the promised land, Deuteronomy 20:16-17 (WEB):
(16) But of the cities of these peoples, that Yahweh your God gives you for an inheritance, you shall save alive nothing that breathes;
(17) but you shall utterly destroy them: the Hittite, the Amorite, the Canaanite, the Perizzite, the Hivite, and the Jebusite; as Yahweh your God has commanded you;
It was because these people's DNA had been corrupted by the hybrid race that the angels had produced - the Nephilim. Satan was trying to thwart God's plan to provide a Messiah to redeem all mankind, so God told the Israelites to destroy those hybrid beings (their human DNA being mingled with non-human DNA). The Messiah's sacrifice of his life in Adam's place could only redeem those people who are descended from Adam and Eve. Anybody who had been born as a result of cross-breeding with angels would not be redeemed.