^Welcome to six months ago.
Also, I didn't think about it before seeing the guard kill the Narn, but energy guns in B5 don't have stun settings, do they? They don't seem to have nonlethal weapons in the security force, atleast none I've noticed.
As it happens, those aren't actually energy weapons in the traditional sci-fi sense. IIRC "PPG" stands for Phased Plasma Gun and it fires what are essentially bullets made of super-heated gas (plasma) trapped in some sort of magnetic envelope. It may look like energy (and it certainly involves a lot of it) but it has mass. The idea behind them is that using a slug thrower on board a space station, ship or inside a pressure dome can cause a breach. While PPG's can burn though metal, they can't penetrate nearly as deep, so much less of a risk. Also, they're better from a law enforcement POV where you'll often want to shoot people without killing them. A wound from a PPG is instantly cauterised, reducing the risk of the target bleeding out and eliminating any complications from shrapnel. Of course it's still lethal if you go for the head or major organs, but that goes without saying.