I think the writers created Euros to be SO clever and SO evil that they didn't really know where to go with it. They built her up so much and I expected a really clever, mind blowing story, but it just turned into X-Men or batman where people were invincible and she was like a cartoon character.
Similar vein to Mary - she was meant to be a wicked, cold hearted killer. But in the Mary-centric episode this season a) she couldn't sell it (partly why people thought the episode was fake in some way) and b) we still weren't shown why she was supposed to be this fearsome assassin. Some of the 'good guys' in Sherlock have done worse on camera. Sorry to say it, but Amanda Abbington is lovely and she's just not a good enough actress to play anything else.
SO are they trying to say a large part of the Moriarty obsession and hatred for Sherlock came from him meeting and being "manipulated" by Euros then?
No, Moriarty probably became aware of Sherlock as a kid, when Sherlock insisted Carl Powers' death was a murder. Eurus just became aware of that - from inside her glass box on an island...
By the way - if all Eurus needed was a hug, and she's now non-verbal and just likes to play the violin, it's rather chilling that she'll presumably remain in the glass box for the rest of her life. Couldn't they make more civilized arrangements? (With these scenarios I always wonder where the loo is... Maybe she's too evolved for that too...)