Whoever wrote that article didn't get it quite right. For example, in the statement:
"They often use descendants of two weapons: the Kalashnikov and its Western military response, the AR-15."
While I know what the author was trying to say, the AR-15 was NOT the "military response" to the AK-47. That was the M-16. The AR-15 is a so-called 'civilian' or 'sports' version of the M-16. Like the AK-47, the M-16 can fire in fully automatic mode, while the AR-15 can only fire one round at a time. While some offshoots of the AK-47 are also semi-automatic (firing one round at a time) an actual AK-47 is capable of fully automatic fire.
As for the idea that the AK-47 and its offshoots are somehow more popular among mass killers, that's a bit of a disingenuous statement since the AK-47 is the most common assault weapon ever manufactured. There are literally millions of them spread around the world, manufactured in dozens of different countries (estimates are as high as 100 million versus about 7 million for M-16's an it's offshoots). Just random statistics would give you the same result, irrespective of whether the weapon is actually preferred or not for some reason, other then perhaps availability and cost (a fully automatic AK-47 can be had for about 1/3 of what it would cost for a semi-automatic AR-15).

OCU