Was Herod's Slaughter of the Innocents Out of Character?

One of my atheist debate opponents claimed that it was. But that is the utter opposite of the truth as we know it from historians. It’s perfectly in character for a tyrant who murdered two possible royal rivals (see the citation below). Herod was no choirboy. According to one secular source:

The first 12 years of Herod’s reign (37-25 BCE) saw the consolidation of his power. He built fortifications in Jerusalem, Samaria and at Masada, silenced all opposition to his rule and eliminated his Hasmonean rivals, Aristobulus and Hyrcanus II, the brother and the grandfather of his second wife, Mariamme. The former drowned in an arranged swimming pool accident and the latter was strangled.

Mariamme met a bitter end as well, and was executed (a la Anne Boleyn, for “adultery”) in 29 BC. The above information was drawn from the record of two prominent historians:

Our chief informant is the Jewish historian Flavius Josephus (37 - c. 100 CE), who devoted most of Book I of his Jewish War and Books XIV to XVII of Jewish Antiquities to the life and times of Herod. Josephus uses as his main source the universal history of Nicolaus of Damascus, the well-informed teacher, adviser and ambassador of Herod.

But that's not all, by a long shot. Dr. Paul Maier, Russell H. Seibert Professor of Ancient History at Western Michigan University, details many further Herodian horrors:

Josephus gives us just a hideous tale of what was going on in the family with attempted poisonings, one brother against another. It so rattled Herod that he actually put to death three of his own sons on suspicion of treason. . . . he killed his mother-in-law — I should say, one of his many mothers-in-law. He invited the high priest down to Jericho for a swim. They played a very rough game of water polo and they drowned him. He killed several uncles and a couple of cousins. . . . 

At one point late in his life, Herod plots to kill a stadium full of Jewish leaders. The plot fails, but what does it reveal about him? . . . [Herod] invites his sister Salome over, and he says, “I want you to arrest all the Jewish leaders in the land and imprison them in the hippodrome, just below the palace here.” (And the hippodrome has been discovered archaeologically, by the way.) And so she does what he asks, and then she says, “Brother, why am I doing this?” And Herod says, “Well, I know that when I die the Jews are going to rejoice. So I want to give them something to cry about.” And so he wants these leaders all executed in that hippodrome, so that there will be thousands of households weeping at the time Herod the Great dies. So is that the kind of sweet guy who could have killed the babies in Bethlehem? Yeah, I think so.

So could Herod conceivably kill a group of young infants (Maier estimated that it would have been only 12-15 infants -- not hundreds --, in a town at that time of about 1500 residents), out of jealousy over a possible kingly rival? Yes; it’s totally in character. No problem!