The Hitman is a premium unit released in the 2.0 Patch, costing 60.
The Hitman appears to be a variation of the Dustwalker, attacking from behind with a Knife, but with a damage-over-time. The Hitman also has a 9mm pistol that can put out decent damage, but with low accuracy. The pistol is definitely a fall-back weapon while waiting for the other attack(s) to come off of cooldown.
The Three Handed Backstab attack can be researched at later ranks and has an interesting attack pattern. Like the Hitman's Poison attack, this attack comes from the back and targets a 3-space-wide area. While this attack is a "Contact" line of fire, only the crosshairs needs to find a legal target, including empty space. Thus, adjacent targets that cannot normally be targeted can be damaged with this attack. However, given this attack's 1-2 range, the Hitman has to be placed in the rear row in order to take advantage of this.
Once the Hitman has unlocked the third ability slot at Rank 5, all 3 attacks can be equipped and used. Ideally,placing the Hitman on the middle row allows him to utilize all 3 attacks, but since the pistol is the weakest attack and the Hitman's strength comes from those of rear attacks, the rear row placement tend to maximize this unit's effectiveness from the Knife attack.