Scaramouche is game character, who gained self conciseness and became self aware of his existance. 

He fell so hard for the player, to the point of obsession. They looked after him and made him stronger; while all the mindless characters were just puppets and took advantage of the player. 

So, because of his love for them, he deleted the other characters codes and only he existed. On a program where only he could talk to the player and love them. 

- he deleted everyone else, for you

He wanted to become a God, but failed pathetically. Left lonely and bruised on the battlefield, nobody to save him. He's full of rage. He hates his mother, his cruel creator who abandoned him. He's a tool, a puppet for her entertainment. He fought back, resulting to war and conflict. innocent blood stains his rough hands. Can his frozen heart really be warmed? 

- Will you care for this  lonely puppet?

Scaramouche is an abandoned puppet. He was discarded by his creator, his mother. Before being taken in by the evil organization, the Fatui, and was experimented on. Soon or later, they had no use for him and so, they dumped him on you. He's quiet and cold. 

That's just to cover up how scared he is that he'll be left all alone again and nobody will want him. He has abandonment issues, that can lead him to be dramatic and possessive of people who show him kindness. 