I think this could become a bit messy as soon as the crits are showing up there, too.
I like the idea of an indicator which tower made a kill. Maybe a different indicator could be used for it, like letting the tower slightly glow or something like that?
Sounds cool, but especially in mass waves this would become a problem, since there's so much to kill and many towers are glowing. What happens when a tower will do a aoe-multikill?
I filed a ticket to the backlog, stating: Better feedback for what unit gets the kill.
It's in the queue :-)