Glower implies a strong negative emotion, but not necessarily an intention to act on it. Even though she glowered at the screen, she remained silent during the meeting.
Pro Tip 2/3
Intensity
Glower suggests more intensity than a frown but less than outright aggression. His glower could be felt from across the room, though he didn't move an inch.
Pro Tip 3/3
Silent Disapproval
Glowering often indicates silent disapproval rather than verbal confrontation. She glowered at the mess her roommate left, choosing not to argue.