He believes that the Canaanites had to be completely eradicated because they had Nephilim dwelling in their midst. These Nephilim were descendants of the cohabitation that took place between the sons of God (fallen angels, aka demons) and human females in Genesis 6:1-4.
One of the disturbing aspects of the Old Testament record was God's instructions, upon entering the land of Canaan, to wipe out every man, woman, and child of certain tribes inhabiting the land. This is difficult to justify without the insight of a "gene pool problem" from the remaining Nephilim, Rephaim, et al., which seems to illuminate the difficulty.
See the video below by William Cooper:
Genocide in the Bible? from Ano on Vimeo.
Because Israel did not fully obey this command to wipe out all of the Canaanites, the Nephilim survived and are still with us today. Missler believes they are the creatures that fly the UFO's.
If you can stomach more of his science fiction, he explains below how the Nephilim are behind UFO abductions:
Lest you think this is some extreme fringe position within Evangelicalism, keep in mind that Missler is a best selling author and a regular on the Christian television circuit. His position is also endorsed by Hal Lindsey, the best selling author (Christian or non-Christian for the decade of the 1970's).