After killing the father, Kahn bitchslapped the son and knocked him out.
Eren was the main cause and also the main perpetrator of the whole public ra*e of Albestros's daughter Cynthia and the tragic death of his son Gerald who died while being unable to save his elder sister who was tormented and toyed with in front of thousands of people and at the end, she committed suicide by biting her tongue and chocking on her own blood.
This time however, since he was carrying somebody with him under watchful gazes, Kahn used the same stealth artifact the three assassins did to cast an invisibility barrier around them in 3 meter radius. His extensions picked up the unconscious Eren and put the dead father's body in his space ring.
He treaded carefully without making any sound using the Silence skill, one of the Archer exclusive skills which completely removed the sound of footsteps when they were doing reconnaissance.