You could arguably do much more damage with a semi-auto vs. a full-auto or burst weapon. You end up wasting a lot of rounds. If you keep cool and take each shot, instead of just spraying, you will take down more targets and be more likely to inflict more damage on the ones you hit.
I'm not sure which would be worse in a active shooter situation, however, I think it would boil down to the training and proficiency of the shooter, as you can control the full - auto / burst weapon with training and practice, however, you can also place shots with a semi-auto much faster and more accurately with training and practice. So, really, I don't know which is worse.
I know when I went through boot camp, they never let us use the burst function of our M-16A4s (we go to shoot full auto machine guns later in combat training), with the reason being that you should place each shot. I also know that they are switching from burst to full auto rifles, with the reason that the burst setting alters the amount of travel the trigger has to go for each shot in semi-auto (single-shot) mode due to how the burst mode is implemented (I remember that - you never knew when the hammer was going to fall when firing as it was different every time).
Anyways, to summarize: if the baddie isn't trained well, probably actually do less damage with full-auto as the weapon will travel high due to recoil and be more likely to jam and the shooter won't have practice clearing the weapon. If the baddie is trained has practiced, you are in a world of hurt no matter what fire mode they have available / choose.
To address some recent posts:
Fou can modify the firing pin on any semi-auto to go full auto by filing it down. (AFAIK there isn't a way to manufacture a semi-auto that can't be modified to do this?)
You can also squeeze the trigger so fast that it's pretty much the same as full auto except with more control over aim.
You can also bump-fire a semi-auto, turning it functionally into a full-auto weapon.
Bump-fire vs. fast semi auto shooter, side by side:
Basic operation, and bump-fire demo:
More bump fire:
Semi-auto (NOT bump fire), rapid fire, good demonstration of how fast you can reload, starts at 1:50, note how fast he gets:
Immediate action (clearing jams) explained:
And yes, you can bump fire a plinker as well (Ruger 10/22):
EDIT: Full-auto AR-15: