I think it boils down to what you are personally used too.
I agree with Ravenhael - I always thought my behaviour was abnormal, until I met other Satanists, and realised I am perfectly normal.
But I don't think that what the majority rules to be normal necessarily is. Let us not forget that the 'majority' does as it's told.
Do you remember being at home with your Mum and Dad before you started school? Everything made sense then. For me, we had a cat and a dog, a TV in the kitchen, 4 living grandparents, My Mum did a 'woman's job' (Nurse) and my Dad a 'man's job' (Policeman).
Everything was normal. Until I went to school and discovered that some people had no pets, no TV, Father a nurse or parents on benefits etc.
On a minor scale this ignorance before encountering other families shows 'normal'.
Does it stand to reason that Garith's armless wonder is abnormal? Not if we didn't know anything else.
I think abnormal in this sense is only what you are not accustomed to.
As far as Mass murder - I think it depends which side you are on. A lot of people forget that Hitler did not have special powers. He was voted into his position because of his beliefs, much like any other political leader. (Except that cheating, scamming Bush.)
If you watch a film, with say a jewellery heist. They'll make out the thieves to be the good guys and the owner of the loot to be 'evil' (By which I mean he'll have a scar and smoke cigarettes.) Which would be the other way around if it were a cop film. Does this mean being a thief is normal or acceptable?
To Hitler, he was getting rid of dead wood. Which I'm sure, after he explained it to his colleagues, seemed perfectly normal. But hindsight is wonderful.
Another example - Christianity. Is it normal for a person to have a delusional belief in a higher power? Apparently yes, if you are a Christian, but no if your higher power is Aliens. They find it perfectly normal
to think their god is omnipresent and watches and judges they every move, but they'll throw a conspiracy nut into an cell and ridicule them.
Society is riddled with abnormalities people have got used to, and normalities they have shun through fear. Make up your own mind what is normal.
I think as with good and evil, Normal/Good you like, Abnormal/Evil is what you shouldn't like!