First off: modern militaries DO get some nice mass executions a lot more often than one would expect. Well worth doing some reading up on that.

Second: Automatic fire is still horrible for that. If your bullets have a high degree of penetration then you are, generally mortally, wounding multiple civilians per shot and are better at aimed shots at clusters of women and children. Or your bullets don’t have a high degree of penetration and you mostly just light up one or two kindergartners whose corpses take up most of the shots. At which point you are, again, better off at firing off a bunch of snap shots.

Third: The actual reason militaries have automatic weapons is for situations where aiming is difficult or less important. Machine gunners at the squad level are expected to fire very short controlled bursts (otpimally single shots) to actually suppress a target when trying to “keep some heads down” so that the maneuver group can flank. Or they are engaging at significantly longer ranges (which is why machine guns often have a larger caliber round than the rifles) where a short burst increases the likelihood of hitting a target. And while it is mostly out of favor, many infantry rifles had burst fire capability or even simultaneous fire capability (either with two barrels or a ridiculously high rate of fire burst) to increase the likelihood of infantry hitting a target by spending more on ammo than training (before realizing it significantly increases the cost of the weapons AND requires more training so that the high schoolers can maintain their weapons in the field). But modern optics, and a decade or two of being the only people with NVGs, rendered that obsolete.