This AddOn shows an image and plays a sound when you get a killing blow. When in a battleground, only killing blows on players are recorded; but all killing blows are recorded outside of battlegrounds.
You can configure the AddOn by editing the options at the top of core.lua in a text editor. This lets you enable/disable the printing of your current KB total to the chat frame and choose whether the AddOn works everywhere or only in BGs.
You can also change the texture and the sound. The AddOn includes three textures: an Alliance crest, a Horde crest and a picture of the Horde/Alliance battleground flags.
The KB total is reset whenever you go through a loading screen (e.g. when you enter/leave a battleground).
You can view the AddOn's source code on GitHub.
Thanks to whitefreli for contributing the KillingBlow_HordeSword texture.