The Murdock boys literally have the devil in them- a demon bound to the family, passed down from father to son. When Jack died Matt "inherited" the demon. He tries to keep it contained inside him, but some nights when he's out as Daredevil he just can't help but let it out. (Make it as serious, scary, or funny as you want. Is the demon bored of all the Murdock drama and annoys Matt when he's trying to be a serious lawyer? Or is he trying to take control of Matt's body to gain his freedom? Just give me Matt with an actual, literal demon inside him that he uses to help him fight.)