It is a religious fervour badge.

Artwork
KIL KIL KIL

A recurring theme in early Rogue Trader art and models was having the letters KIL painted on everything. There was an explanation - it was a badge awarded by the Imperium.

Imperial symbols
From the Imperial Army section of the Rogue Trader rulebook

The real mystery is - why did the Terminator in White Dwarf 109 have REVENEBO written on his shoulder pad?

Pencil artwork
Terminator by Pete Knifton