This article reviews Freud's use of the concepts of activity, aggression, and sadism between 1905 and 1915. We propose that in Freud's conception of the psychic apparatus the notion of activity works, implicitly or explicitly, as an element that is able to set off movements and processes, giving them a specific direction and a role. However, as it is found in the description of instinct dynamics, in the genesis of the task of representation and in the orientation of defenses, the notion of activity also expresses itself by the tendency adopted by the subject in his relation to external objects. When applied to the subject's interaction with his environment, activity gains certain attributes: it becomes aggression, a derivative that is no longer the same as its element of origin, but continues to bear its mark. Aggression becomes mixed, fused with other qualities of psychic life: it becomes excessive, pleasurable, repetitive; a compound that, between 1905 and 1915, Freud identifies in different formulas that imply sadism. From a selection of Freud's published texts from that period, this article presents and discusses these relations and offers some indications for a psychoanalytic theory of violence.