Speaks with an accent. Your example of a mastermind was Loki, so I figured the Mastermind wouldn't be obscure. The fact that the attacks aren't American implies a sort of global awareness. The attack on the SHIELD safehouse for weapons makes me think of a military leader. Doctor Doom wouldn't need somebody else's supertech. So I figure Red Skull or Baron Zemo, but Skull's more prominent, so I guessed him.