First of all, i think Karthus is a awesome champion. Kill stealing isn't bad, I mean, it began on public games where there's no teamplay.
If a Karthus in a WCG gets a kill with he's ult it's GREAT. Karthus is an awesome mage and can take care of almost every champion with he's wide range. He's a farmer too, so why not letting Karthus get farmed?
If you kill, it's one point to your team. If Karthus kills a lot, you should LET HIM because he's gonna help you win the game and you get the assist so.. you get gold too!
I played Karthus for a couple of days now and he's good to start a battle with his ult and to finish it. I got triple kill with the ult that was pretty fun and i prevented the enemy team to escape in the jungle.
You can use your ult when they are running away and your team isn't gonna get them, so you do. But don't use it for killing someone who's 'already' dead, not because you'll KS but because you're gonna waste your ult and it has a really long cooldown.