The old adage states that ‘justice is blind’ and in the case of the Megara it must be true. For an enemy that can’t be seen properly and are sentient, you’d get the feeling that you would constantly be looking over your shoulder if you felt even a slight presence one was near. The Doctor originally met the Megara in The Stones of Blood and they appeared in one of those instances where the Doctor mistakenly releases the villain. It was only through a big of trickery and double bluffing that he managed to get them from turning him in to a pile of stones.
So who are the Megara?
The Megara are a bunch of floating orbs that travel around the universal handing out judgement to those they deem as having broken Galactic Law. Think of them as away more advanced version of Judge Dredd, only they didn’t really uphold the law.
What do they do?
They’re a floating courtroom essentially. When the Megara were made upholders of law by a galactic federation, they found the federation in contempt of themselves and destroyed said galaxy, making the laws they uphold really just an excuse for killing anyone they don’t like. They also transport prisoners form one planet to the next, which is why they cross paths with the Doctor.
Are they dangerous?
Well unless you love the idea of being turned to stone, then yes they are dangerous. Yes if they believe someone to be guilty they’ll have no hesitation there and then of turning you in to a pile of rocks. The best guess as to why it has to be rocks is because of their name’s origin. Megara is the name of a character from Greek Mythology, whose children were killed and them entombed in heroons. Heroons are stone erections placed over a person’s tomb.
Did they like humans?
Of course not. They see humans as being inferior, which is slightly justifiable when you can fly through the air and essentially contain no mass. They also believed that humans were just too dumb to see through lies and get right to the truth, which is probably what makes them slightly short tempered.