Eren could only laugh in his head after listening to Nina's warnings.
He had been nothing but prudent in handling these matters. That's because he had no choice. The situation was grimmer than Nina thought it to be because of his artificial half-blood status. He wouldn't even be welcomed by the cultists if the exact details about him were known to them.
Nina also felt that the side effects the butcher received from using such an otherworldly Ability were too shallow to qualify as side effects. It was as if these Abilities were made for him in the first place and not granted to him through some ritual.
Eren released a lungful of smoke through his mouth while looking at the morning sky. He caressed Nina's back to assure her that there was nothing to worry about.