How do believers explain this slaughter?
I'd looked up what little WT had to say about this and other accounts of the slaughter of innocents a long time ago, about the only statement I recall is that these people had no special right to life--it was as if WT was justifying it by saying, "people die all the time, so this is no big deal that he would slaughter thousands of his own chosen nation."
And although it was supposedly wrong in this account to conduct a census, there was no specifically stated law against it. There are plenty of times in the bible that the populations are mentioned. This all just sounds like an excuse for the angry desert sky-daddy to go on a random murderous rampage against his own worshipers. Nice.