Lyaeus could be seen in extreme anger as he was just attacked so suddenly without any warning. If he did not tilt his body reflexively earlier then it would have been his head rolling off. The neck part is the most vulnerable part of the player body and is considered the easiest to hit off guard. This is the reason why assassin players can kill other players in one or two attacks because they always aim for the vital points.
Eldritch wanted to have Lyaeus' head cut off there but he is slightly impressed that the latter actually dodged that attack. It was one of the attacks that he honed and wanted to use on the people that he wants revenge after. He also honed it to a high degree to create an attack that could kill out the weeds of this game as he called it.
