The Royal Mercenaries were a highly-trained group of people, dedicated to dangerous tasks inside the tower and outside the capital. They take years' worth of harsh training to hone their skills and mind for the horrors they were bound to face.
So, by the time they become full-fledged mercenaries, they are all ready to face death and even laugh in its face. On top of that, with years of work, they gather even more experience and face extremely dangerous and extremely terrifying situations basically on a daily basis.
So, if there was one group of people capable of assessing a situation, they were that group. Yet, these seasoned warriors simply watched in horror the scene of Arthur brutally killing every Yeti in his path.
If they had to describe it in one word it was… Nightmarish. They weren't watching a human, that was a monster in human skin, taking pleasure in dismantling these creatures in the most brutal way possible.
