They didn’t do any due diligence and completely turned their backs on the possible assassin. The hurt feelings of one Butler if they take the princess away from an accused assassin mean nothing compared to the risk of it actually being one.
The butler was in proximity of the target before, meaning whatever security measure the have were already taken (if any)
Which is to say the assassin, worthy of his skill, was 'trusted' as of when Noel jumped in.
To expect them to trust Noel, at this point unidentified person, would means you expect the guards to paradoxically be both wary of the butler enough to trust stranger's warning AND trust him enough to let him get close to the princess.
And if you're expecting them to be paranoid enough to trust no one, then the fact you expect them to immediately trust her is clearly some bias thinking on your side (if they're REALLY paranoid of everything, they'd be paranoid she's another assassin, using either innocent person/first assassin to make them think she's on their side)
It's not like criminal pretending to be the victim/good guy is even rare.
Nor is having a battle with a mage of at least some skill with the princess right there as an easy potshot a good idea. The competent thing to do would have been to have at least one or two designated to move the princess away from the theoretical danger WITHOUT possible assassin A coming with her.
We don't actually know what they were doing on the side.
Though consider Noel who could move faster than they can react still got eventually suppressed by the sheer amount of shadows, I highly doubt the guards could just push through to safety.
I'm not sure why you people think every 'competent' guards have to have power on par with THE elites of royal court mages as well as assassin who's using priceless artifact.
If you think the guard should totally be able to handle that, either you're underestimating the power level of Noel/the assassin(or rather the artifact he had) or greatly overestimating the guards.