Although Ivy had her fair share of bloody scenes, seeing the way Ciel killed the man still made her shudder.
Ciel walked away without giving the corpse another glance. He didn't even check to see whether Ivy was following him or not. He was busy trying to calm the bloodlust that was trying to overwhelm his senses.
The whole base reeked of a bloody smell and Ciel was intoxicated by it.
To be perfectly honest, Ciel had always wanted to test his new found strength against the elite soldiers of Memento Mori. Since they would be the ones he would be dealing with in the future, he wanted to know the extent of their combat abilities.
Even though Ciel had killed a man for the very first time in his life, he didn't seem to feel any guilt nor suffer from its backlash. He found this weird and tried to rationalize it by telling himself that this was payback for what they did to him.