"We sieze control FOR THE GREATER GOOD." Dumbledore says this in a letter to Grinewald. Although he was young, he did believe this. That phrase can be the most scary thing that you will ever hear. If Taking over the muggle world is for the greater good, I am highly disturbed. What is quite so good about ruling over other beings. People that can use magic are just like people that can't use it but with powers.
What exactly does for the greater good mean? Is it for the well being of everyone or is it what you think that is? My opinion is that the greater good is a benefit that everyon likes. Some may not agree but I think that killing Osama Bin Laden was for the greater good. He was a tyrant and nothing bad happened because of it.
There is one huge flaw in for the greater good. Bad people think they are doing things for the greater good. Hitler thought that wiping out the jews was for the greater good. Was it? I think that Voldemort Believes what he's doing is for the greater good. It's all a matter of opinion. Whether that opinion is good or bad, it is not easy to define what is for the greateer good.