The Widowmaker SMG is a common SMG modified by the Omar. Its last known user before Alex D (if the player finds it) is an unnamed SSC Guard who was presumably killed in the Archology after the Templars took control of the facility.
Be cautious when using the alternate fire because of how much ammo each shot uses. It can be easy to run out of ammo if constantly using the alternate fire in battle. Surprisingly, the Ammo Scavenger mod does not change this.
As with the spiderbomb, the player should use it as a distraction when fighting enemies. Fire a spiderbomb, the shoot enemies while they're distracted by the spiderbot.
The Widowmaker is useful for medium-range shots because of its increased accuracy.
The same mods for the SMG also work with the Widowmaker SMG, with the bonus that shots will be more accurate than the normal SMG.