Well, there is also the way Hiro told them what he did. It's a big contrast. When he told Andou it was like "Yes, I kill people, it's fun and I don't care". When he told the girl it was more like "I do bad things to people :("
You got a point there, but he could have tried
Hiro killed children. Children in their own freaking home, unprovoked. You're expecting way too much from Andou.
You have to bear in mind that as the viewers, we see way more than any individual character. All Andou knew was the person who used to be his friend was now a murderous machine that lacked any apparent empathy. Who knows what would have set him off? Andou wasn't some hero, he was a kid scared for his life.
Let's be realistic here, Andou was already a hikikomori after the trauma of constant bullying, and you expected him to somehow be able to talk down a serial killer he once kind of knew? If I sat you down in a locked room with an unbound convicted murderer and handed him a weapon, would you, defenseless and vulnerable as you are, really have the guts to risk offending him? How much sweat and other liquids would be drenched on your soon-to-be-discovered corpse?
Andou gave up on Hiro waaay too early. If only he had supported and be there for Hiro like the girl did, he probably wouldn't have killed anymore. I know he's a mass murderer but I can't help but cry for him. （/TДT)/