I asked said player.
He said JU was for Junker, as in JU-88/87 etc.
If he is being honest, then he's an thoughtless idiot. Whatever the case, that is an unacceptable handle.
If anyone thinks that "JUKILLER" is harmless or doesn't offend, go ahead and put a bumper sticker on you car that says "JUKILLER" and park it in a synagogue parking lot overnight... Double check your insurance deductible first.
My regards,
Widewing