There are two reasons he became cold. The first is that you can't show too much affection to deranged people. They will become addicted to your care and love and they won't be able to live a normal life. They will effectively turn into leeches, who just consume your love to fuel themselves. In such cases they need to be left to fight it on their own.
The second reason might just be Mizutani's arrogance at the time. He was saddened, thinking that despite his best efforts, he couldn't help the kid recover. Basically, "if you wanna use that method.. go ahead.. you don't need me anymore. Everything I did was useless at the end."
It might be both of these reasons combined, I can't tell exactly.