The demon spoke through the man who was possessed by an evil spirit (1:24). In the Gospels, demons are often associated with disease (9:17, 27; Matthew 17:15) and self-destructive behavior (Mark 1:26; 5:2–5; 9:17–18, 20–22).
Demons know who Jesus is (1:34). They constantly identify him as the Holy One of God (for similar phrases see 3:11; 5:7).
In saying us, the evil spirit spoke on behalf of all demons. It recognized that Jesus has full authority and understood that he had come to oppose and destroy evil.