When do you think it is unfair or deceptive to try to use emotions to persuade people

It depnds on your motivation. If it's for personal gain, that's called manipulation, and is extremely deceptive and unfair. But if your simply trying to appeal to someones emtions to get them to benefit a good cause, like showing them one of those videos of someone in the military coming home in order to persuade them to donate to a cause that helps veterans, I wouldn't really call that unfair or deceptive since they know exactly what you're trying to do. So it's deceptive and unfair when they don't know what you're tring to achieve and that you're trying to achieve anything.