My take on this is his job is done. He found two kids who can actually see between right and wrong, and he's entrusting the future to them quite literally. He was probably waiting all that time for someone like that to show up, and had probably helped a fair share of people, but they were all broken like him. I mean the very nature of his setup is oddly difficult for one person to manage. Someone has to watch the screens to tell him where everyone is otherwise he's hampered purely by eyesight. Having the girl release the man was sure to set him and his crew on a proper warpath with the guy, which meant it was time to execute his final plan and take all the bad people out at once, himself included.
He was also probably oddly sentimental with these two even if he didn't show it, they appear to look a lot like his children.