I got hit with a -900 combat penalty for killing someone earlier.
I didn't see a warning, but my first hit killed him, so that might explain that.

Either way I thought the update removed the XP penalties altogether, so I'd assume this is a bug.

This does not affect just you. During the Cap Ship battle earlier today I killed a player twice, after downloading the update, and lost 1200 XP both times, without receiving any warnings whatsoever.

Another player reported losing 1100XP for killing the same person.

I lost 1700 for killing someone. (He wanted to fight me)

wierd.. Did this just start happening recently? cuz after they fixed it for the second time I've not lost xp from killing anyone, but then again I've not fought any 0/0/0/0/0s, although I've fought a few with like 1/2/1/0/0

As of patch 1.2.5.3 (22^nd or 23^rd of August) they removed the XP penalty for killing lower level characters. They intended to in 1.2.5.2, but didn't actually apply the change.

http://www.vo-wiki.com/wiki/index.php?title=ChangeLog#This_is_Vendetta_version_1.2.5.3.