That guard looks exactly the kind of guard who might, if not stab, at least be willing to poke a princess with his weapon, as long as it’s not the specific princess to whom he swore allegiance.  
Civil wars have been started over lesser things than cake-theft and the resulting guard-on-guard battle when the sun-guards want to arrest the night-guards for protecting their princess’ cake with traitorous levels of force (so, any force at all, against Celestia)

oh yes, she’s just… raising the sun, not raising any sort of cherry on a certain sister’s special reserved cherry chocolate blueberry cake.

uh-uh, bitch

Celestia: “Ooh, forbidden cake! Best cake.”
 
Guard: “If you eat this cake, Princess Luna will get mad.”
 
Celestia: “Oh, I’ve got a plan for that. Remember my student?”
 
Guard: “Not Nightmare-mad. Just… mad-that-you-ate-her-cake mad.”
 
Celestia: “So? Live life dangerously!”

Before that shift, he was but a mere colt, used to playing schoolyard games.
 
Afterwards, he was a STALLION, and all the mares flocked to him!

“Yeah, that’s right. Cherry floating away. Into my mouth. What are YOU going to do about it, fangs? That’s what I thought. You just stand there and scowl at me. Om nom nom, chump.”

@Shadowking58
 
@Background Pony #F9FF
 
@J4m35
 
@robinebra
 
Maybe this Guard angered Luna somehow, and this is her version of being reassigned to Antarctica: being forced to stay up all day, alone, to perform the impossible task of guarding a cake from Celestia.

The guard is like: “I’m not paid enough to deal with this s*it”.

There is a special circle of Tartarus reserved for you, Princess.

LOL

Guard: “Why do I have to put up with this sh*t?”

@Background Pony #30E4  
i kid, man, i kid

Yeah okay chill the fuck out.

@Background Pony #30E4  
Because i didn’t think of the second post until AFTER i posted the first, otherwise I would’ve posted both in the same message OK?!?!

Why are you replying to yourself?

@CranberryRodent  
“yo it’s okay I’m the princess

STOP RIGHT THERE, CRIMINAL SCUM!

Thus, civil war begun to ravage equestrian lands.

was it established that the horn glow was for our benefit?

“Your Highness, please don’t make me stab you.”

That guard is quite aware what is going on.

yet another Andy Price’s emulator :P

That Royal Guard looks pissed

And then: https://www.youtube.com/watch?v=YJpwfvpTPWg