Every day, the priests urged Rameses to lay the law's fist on Moses. They urged Pharaoh to just torture, imprison, or execute him. If Moses were any other man, he would have been dealt with long ago. But this man who unleashed a foreign God's scourge on Egypt wasn't a stranger. No, this professed messenger of some God was still Rameses' brother. No matter how many terrible plagues he brought down, Rameses could never, not for all the past Morning and Evening Stars, bring himself to order Moses' execution, let alone torture.
Otherwise, he'd never forgive himself as a brother.