Perhaps it is our motive, in a sense. Our killer was apparently motivated by embarrassment, so if they were truly caught unaware and forced into murdering Gilbert as well, as twisted as it is, I can see how they would want to avoid the idea that they were caught aware. It's embarrassing, or so it would be to someone motivated to this much.

I can't claim to truly understand it, nor do I want to, but it could have been meant to spite that notion.