Kiritsugu's childhood certainly ended soon and abruptly. I think David nailed the reason why Kerry killed his dad. He is also his father's son, so the same sort of a mage's decisiveness is within him, allowing him to do it. He's simply not interested in the Root, like so many other mages, but justice. It really explains how he can be so ruthless and no trick is too dirty for him if his first kill was his own dad.