ZSS has great kill potential; her kill moves don't kill particularly early but they're all reliable and kill at reasonable percents. A fresh back air near the edge of the stage will kill at like 110-120%, same goes for forward air (2nd hit) except it kills a little earlier but is harder to land. Both of these moves are strongest at the tip of the foot btw, so make sure you do that if you get a dsmash at kill % and you want to finish them off with one of these moves, hit with the tip of the foot.
Fresh sideB near the edge of the stage will kill MK at like 95
A fresh uair will kill a grounded MK at like 145
In terms of reliability and what you're most likely to kill with, I'd say bair --> uair/sideB --> fair, assuming these are not following a dsmash
Our downB spike is really strong but obviously very risky as well. If you hit with it from about eye-level with the stage, it'll kill MK at like...30-35. Obviously the higher up he is on the stage, the more % it takes for him to die.
Our upB spike is safe but hard to hit, and it's pretty weak so if MK's at eye level with the stage it won't kill him til about 100%.
Also, if you get a grab, and you don't throw MK, MK will automatically air release (unless he breaks out while you're pummelling). ZSS gets a lot of guaranteed followups out of air release on MK, including dash grab (so we can chaingrab him), running usmash/dash attack (dash attack is good at low-mid %s cause it combos into other stuff so it's good damage), uair (so yes, if you get a grab on MK at kill% you can get a guaranteed uair to kill), fair (really hard), nair (fairly hard)...and I think that's it.
You'll want to practice air release --> uair but after you've practiced it, it's fairly easy. What this means is that if you get a grab on an MK near kill %s,you can pummel during the chaingrab (ie pummel once or twice --> air release --> regrab --> repeat) to deal damage and unstale your uair, and then kill with air release --> uair.
All of these %s mean MK is at this % BEFORE you hit him, so if you hit MK with a fresh bair at 110 and it does 13%, it'll send him to 123% and probably kill.
If you hit him at 97 and it does 13%, it'll send him to 110% and won't kill.
Generally, I keep bair fresh until I know the opponent (of any character) is at kill %, and then use it to kill. Uair is usually stale for me since I use it to juggle to rack up damage, so I don't usually kill with uair though. Against MK however, since he's fairly light and we get a guaranteed uair to kill out of air release, then you can use both uair and bair to deal damage without really having to worry too much about your kill moves being stale (especially with the pummels you get during the chaingrab)
Also...at high %s (like 100+) you can get triple dsmash --> uair/bair, but the timing is pretty tight (especially for bair, I'm not even sure if it's guaranteed)
I think that's it. Once again, excluding dsmash followups, bair --> uair/sideB --> fair2 --> upB --> downB in terms of reliability
