The Christian God, which (judging from his writings this year and previously) Hendel does not accept as his own, teaches the primacy of the Golden Rule, which inherently forbids not only cherem* genocide but also both shooting abortionists and abortionists killing babies, and even Texas drivers following ten feet behind another car at highway speeds (which statistically is the same thing).
The authority of the Christian God's morals over Islamic fighters committing cherem on "infidels" today is implicitly acknowledged by those terrorists themselves when they hide their faces in shame as they commit shameful acts in the name of their god. Whether in shame or in pride, whether their nominal co-religionists approve or disapprove, they (not Hendel) still have the last word on what their god requires of them. If their murders are sufficiently offensive to the other deities (and non-deities ;-) competing for supremacy in today's world, "survival of the fittest" is a reasonably good way for the rest of us to know which god is indeed greatest. Hitler's (atheistic) god is dead with him. Stalin is dead with his non-god. Judging from the flow of people escaping their domain, ISIS may not be far behind them. The Christian God (so far) has the best survival rate (see also "Man vs the Bible").
As for the Christian God commanding cherem on people and cities more than 3000 years ago, I do not have enough information to adequately evaluate His purposes within its religious and cultural context, and (again from his writings in BAR) it would seem that Professor Hendel understands even less. We (he and I both) do understand the religious and cultural context today. Whether Hendel's god (or himself, which is probably the same thing) denies cherem against ancient Canaanites or against non-Muslims in Paris today, while permitting it against unborn babies and against safe drivers, or not, is for him to say, not I. At least the orthodox classic Christian God I claim as my own is well-defined in a document open to public inspection (although not many actually do). Private ("separation of church and state") religions or "non-religions" (same thing, but without acknowledging the supernatural) are not open to inspection and thus cannot be criticized nor redefined by persons outside that particular faith.
The only protection innocent citizens have from cherem* perpetrated by private religions is the destruction of Jefferson's "wall of separation between church and state," where the government overrides the religious practices of those people not aligned with the state religion. When that happens, some religions (in the USA that would be atheism and LGBTQ) are "more equal" than others (Muslims and classic Christians and unborn children of color). The USA is still way ahead of whoever is in second place, just not as far as it was 100 years ago.
* Cherem (pronounced
like "harem") is what God told Israel to do to Jericho: destroy everything
and kill every man, woman, and child, and every animal. It is what Samuel
told King Saul to do to Amalek. Those were specific commands given to Israel
thousands of years ago, in circumstances we don't know very much about,
except that is no conflict with the Commandment "Thou shalt not murder"
(which is a different verb).
Jesus told us Christians to "love your enemies." Our Christian cherem
is coming (some time in the future) but our God is greater than Allah (also
greater than the non-god of the atheists), and He can fight His own battles
without our help. We get to watch and cheer, but not actually get blood
on our hands.