First of all there are no leaders, no machine guns, no morale checks or broken units. Once those are added you can call it Squad Leader.
In the original rules, it is very rare that you get a KIA when firing. It's more likely that you'll get a "Morale Check" where you roll the die against the unit's morale number. If you have an unfortunate morale check roll, then your units are "broken" and are required to run for cover away from enemy units. Leaders stacked with units help with the morale checks, and can "rally" troops to get them unbroken.
Machine guns are used to create devastating fire lanes with their long range and penetration effects.
That's just the introductory rules. Successive modules added such things as panzerfausts, smoke, tanks, americans, minefields.... until we have "Advanced Squad Leader" which is an incredibly detailed and complex game system.