I pretty much hate the Executioner as a Striker. It fails at it's roll, unless you charge optimize, but even then is outdone by all it's Essentials charging brethren.
It's "striker mechanic" is that it adds 1d8 damage to its weapon attacks and it can base MBAs on Dex.
A Thief can use Dex for MBAs and starts off with a +2 damage to weapon attacks (which eventually increases). It then adds 2d6 extra damage (increased to d8s via feat). And can be charge optimized just as well.
Also, the Thief is more accurate as it has a weapon talent that adds +1 to hit. The backstab Encounter power, while not adding 3d10 initially, improves the chances of hitting. I mention all of this, because hitting is almost always more important. Also, the Thief gains more uses as opposed to higher damage (they gain some extra d6s). But really that's more +3s to hit. In general these are just more effective than a hard to gauge Death Attack (which should have had a cap 5 HP or so higher or at least trigger while adjacent so your allies could gain a benefit).
The poisons are rather weak as well, but I've ranted enough about how much this class disappoints me mechanically (ok... a little bit more).
Honestly, they should just have gotten the "add second ability score to damage" benefit to at least balance it with other classes. Its poisons don't exert the control of other striker/controllers or add enough DPR to stay competitive.
And the classes flavor is easily accomplished by having a Thief multi-classing into the Assassin to pick up the Ki Focus. I'd rather play the even WORSE damaging Original Assassin because that at least has fun flavor.
To be honest the Ossassin can be tweaked to outdamage the Executioner. The main difference will be that it will have to spread the damage over several targets. Personally I don't really like the executioner's poisons. If they had dealt ongoing poison damage then it would have been different as the Venomed Hand PP would have given them more nova potential.