Brian Thomas//Hoodie

The main motives for Hoodie's aggression towards Alex
Revenge for past events

Hoodie attributes her symptoms ("The Disease") to Alex's actions during the filming of Marble Hornets.

He blames Alex for his problems - distortions of reality, coughing fits, loss of control.

His own condition. Hoodie is experiencing symptoms similar to those Tim is experiencing: coughing fits, distorted reality, and memory lapses. Pills help him temporarily stabilize. In entry #73, Hoodie reveals his stash of pills, suggesting he's stockpiling them.

Controlling the situation. Stealing Tim's pills is a way to weaken the enemy. Without his medication, Tim's condition worsens, making him less mobile and predictable. Hoodie uses this to gain an advantage in the confrontation.
Hoodie's primary source of pills is Tim Wright (aka Masky). Several episodes show Hoodie specifically targeting Tim's medication:

He constantly wears a hooded sweatshirt that hides his face. He is often depicted wearing a yellow sweatshirt (this color has become his signature feature in fan interpretations).
In the image, Hoodie wears a black mask with a sad smiley face drawn on it (it is made from a T-shirt).
His movements are smooth and stealthy, making him look like a professional spy.
Prefers to avoid direct confrontation and operates from the shadows.

Silent and unsociable, he prefers to communicate through actions or notes.
Unkind to others.
Mysterious - his goals and loyalties remain a mystery: sometimes he helps the heroes, but at other times he becomes their enemy.
Manipulates - creates confusing situations that make others doubt the reality of what is happening.
Cold-blooded - ruthless and calculating in his actions, it is difficult to catch him off guard.