What you describe sounds like a Surprise Round attack using a concealed dagger by the Thievery skill, which can be drawn as a free action or not depending on your ranks in the Short Blade Fighter (rank II draws as free action).
Since this is a surprise round, he would automatically gain +1D, and since his opponent won't act this round, he could use the Reckless Attack (greater) action for another +1D on his attack if he can draw as free action, or draw his dagger as a (lesser) action, keep it hidden using his Thievery skill, make the Aim (lesser) action to gain +1B then make a regular attack (also lesser action) at an unsuspected target.
If you consider that the average person has Awareness 2, that means a passive Awareness of 8. So a character with high Stealth could easily pull this off against the great majority of victims. For instance, Stealth 3 (Sneak 1B) averages a result of 12, more than enough to trick most people.
Keep in mind that each degree of success on his first attack means he does the weapon's damage worth of damage to his target, so the more bonuses he stacks on his first attack, the higher the outcome will be, possibly resulting in instant death to his target.
Some targets might be dead by a single injury, as many people have only two ranks of endurance, meaning that every 2 points of damage they suffer an injury or wound. A Stiletto will do Agility damage per degree of success, so a highly agile character could dish out a lot of damage on a regular guard. The weapon also has Piercing 2, ignoring the first 2 points of Armor on his target.
Knock him out instead
Despite what we think, instantly killing someone on a single hit is not easy, and people train years to be that deadly. An alternative to his sneak attack is to actually try to use the Knockout (lesser) action on the target, which requires an attack (with that initial +1D) and that will either stun and apply a -5 Defense penalty on the target for the next 1d6 rounds, or automatically knock him unconscious with two degrees of success. Being a lesser action, it means you can draw your weapon or aim your attack, as necessary.
If the target is stunned, they will almost certainly take an additional injury/wound on your attack due to the penalty on their Defense score. If the target is knocked unconscious, then they are at your mercy.
Take Short Blade Fighter talents
Back at these, they are very useful for assassin types. The first talent adds +1 to the Piercing quality of your weapon (or adds Piercing 1). The second talent allows you to draw the weapon as a free action, as explained before, and add the number of bonus dice in the Short Blade specialty into your Fighting test result. So a character with 3B would add +3 to the result of all Fighting checks, which increases the potential of an additional degree of success.
And finally, the third talent adds the number of bonus dice in the Short Blade specialty to your damage result. So that same 3B would add +3 to your total damage per degree of success, which can potentially one-hit a lot of people.
A character with Fighting X (Short Blades 3B) and Agility 3 would attack with +3 and add +3 to his damage, so his normal Stiletto damage would go up to 6 (from 3), ignore 3 points of armor, and could be draw as a free action.
The Wounds mechanic can save anyone attacked by an assassin. In fact, that is the exact example given in the book, an assassin attacks a character for 20 damage, they take a wound to avoid that damage.
This rule is true and valid for all player characters. However, NPCs only use that rule if the GM decides so.
The game splits NPCs into three categories: primary, secondary and tertiary (see page 214).
Primary characters use all the rules that PCs do, including injuries, wounds and Destiny Points, as they are important to the story. Those are villains, masterminds, family members, etc. And may show up fairly frequently in the campaign;
Secondary characters are simple, they can take injuries, but cannot take wounds, nor have Destiny Points. Some wont have benefits or flaws. Those are short term enemies, lieutenants, and characters that may last only a few sessions, rarely longer than a single story;
Finally, tertiary characters are mooks, minions, NPCs that you rarely will name. Those are minor servants, guards, bandits, thugs, etc. They cannot take injuries or wounds, none have benefits or flaws, nor Destiny Points.
So, the result of our assassination really will depend on the character being targeted. A primary character surely can survive a strong attack, others, not do much.