I love Ninjas and played a lot of them for various purposes but never really built for Deathstrike as a primary part of the offense. How it works is you hit a target with it and if they move more than 5 steps they take direct damage (which yes, does mean it can be countered by them not moving, if they don't move they take around 10-15 damage) I think of it as the five point palm exploding heart technique from Kill Bill vol II (just me?... ok then). The damage is scaled based on your Ninjitsu/Hiding/Stealth skills and if you used tracking to track the target. I don't know if the damage calculation has been capped or changed but it used to be the case that a tracked target could be hit with a Deathstrike for 60 damage, most of them time though you will do about 40, I've only ever played Stealthers with 100 Hiding/80 Stealth and usually ranged (which doesn't work for it) so never bothered with DS.
Because obviously you want them to move 5 tiles combining it with Disarm is a good idea as not many people will want to stand there getting hit. Also because you're Disarming them you're probably better off having Swords and using Pick Axes (Disarm/Double Strike) so if they do stand there you can get more damage in.
I don't know what difference 80-100 Stealth makes for DS but if it's not much I would stick with 80. It does mean needing Med armour to Stealth in though.
If you're going 4/6 Chiv, I would definitely go 120 Chivalry.
I probably wouldn't bother with Resist at all and have Tactics at 90.
Other things I would consider picking up are Alchemy (100), Poisoning (80), Med (whatever left), Heal/Anat (minimum 80/80).
All that said I'm actually in the process of making one myself which is going to be; 120 Fencing, 90 Tactics, 80 Poisoning, 120 Ninjitsu, 70 Healing, 80 Anatomy, 120 Chivalry, 40 Parry. With suit all 75's, 55 LMC, 45 HCI, 20 DCI, 4/6, 25 HP, 100 RPD, 20 SSI, 10 Healing, 20 Parry. Not using DS at all but a 4/6 Chiv/Ninja all the same.