This is my node setup to make my lightsaber glow. While rendering the image is clear and looks fine. At the end of the render when it adds the glow effect to the saber it puts a haze over the whole image like this.
The glare node has a threshold effect to isolate the brightness value that gets a glow. But it could be that your scene is brighter than the light saber. Perhaps put it on its own render layer yhen apply glare separately. Finally comp that ovee the room.