I often see other people do that, something like "XYZ starts aiming at you". But when I fight multiple people, I can't seem to do it, it just echoes "One battle at a time, please" or "You do the best you can!"

First, they have to be aiming at YOU with their melee attacks.

Then, you type "hit Fred" or whatever his name it.

If they aren't already hitting you with their melee attacks, you can't redirect on them and can only hit them with skills and spells and communes.

So typically, you might bash someone to draw them into combat, and at the same time lag them long enough for you to then redirect at them before they flee.

They have to be attacking you. Don't know why you can't use kill, but that's just what you got to do. Learned it the hard way.

Murder command will attack NPC or PC.
Just maybe so you don't accidentally use kill 1. or something? I don't know, but I imagine typing murder might allow you to redirect, though I always use hit so never bothered to try.