Whatever happened to the fake death command from the original UT? I used to use that all the time when I got a phone call or had to go AFK for a minute or two.
Was also fun to do it underneath a pickup like the big keg o health and stand up when you see someone coming and give them a surprise.
Or occasionally I would fake death so I could spectate a teammember to see where they were.
That said, whether I kill someone idling/afk depends on the situation. If a new map was just recently started, I don't even take a pot shot at them because I know they're probably still loading the map. If they're still there five minutes later though, I may toss grenades at them or get an easy telefrag.
For the most part though I avoid killing idle/afk people even though I hate that they're idle/afk and screwing up the team balance worse than it probably already is. The only thing more lame than being afk/idle in a game is being the kind of person who will run around and kill those people over and over again for easy points.
If I can tell someone is typing something I try to avoid shooting them and may even stop and apologize if I do hit them and its someone I consider a respectable player.