It would work like this: LMB Slice attack, RMB Stab attack. Also has new amimations
Pros:
On kill, you get HP of 25, it is like a Health pack that overheals.
Guarantied MINICrits on back STAB. So a bit of Spy tactics are in order.
Increased damage x%, 15 to 25, maybe?
Has a taunt kill of slashing and then stabing combo to the unlucky enemy.

Cons:
Slower firing speed by x%
Shorter reach(it is a knife after all)

Edit: You get HP on kill instead of the base health bonus.

Should this be a Knife that gives you HP?