[spoiler alert] Please do not read until you watch Episode 11 of the Anime
At Episode 9, and Episode 10, when Rem uses her horn to engage beasts, it seem that she cannot control herself. At Episode 10, She also attacks Subaru and Ram. Subaru has to hit her horn to bring her out of madness.
Episode 10 at 19:47
However, in the flashback when Ram and Rem were young, none of them become crazy when they use their horns.
Episode 11 at 1:26
Episode 11 at 2:17
Could anyone explain to me ? Does madness is the side-effect of using horn, or there are any other causes ? Because as I see, it happened to Rem in the forest with mabeast (demon beast) only.