Adam had become desensitised to killing and with a characteristic that enjoyed and desired to kill, it was something that had become ordinary to him.
He had killed thousands of beasts in the dungeon and already killed a few people in reality, but he didn't feel guilty about it in the slightest.
Adam believed that even without the Bloodlust Characteristic and Hunter's mind, he still wouldn't have felt guilty, as when he killed in the dungeon, he was killing beasts, that although seemed real, would disintegrate into pixels and were attacking him anyway.
While for those that he had killed in reality, he felt that they deserved it and didn't regret killing anybody that he had killed.
Life is the only thing that was given to everybody equally and was the most valuable thing, that from what he knew, was only given once. Because of that, taking someone's life was a big deal, especially if they were innocent.