Power Word Kill is a first-level spell

The human body is resilient, but not nearly as resilient as anyone would hope.  Consider all the myriad ways it may be  broken beyond repair by accident and happenstance; while evolution has seen fit to be charitable and not make our weak points z-targetable, that’s all we’re going to get.

Now introduce an evil wizard to your consideration.
  
All those elaborate power systems? Irrelevant.
 
Special bloodlines? Worthless.
 
Fireballs, lightning bolts, magic missiles? The tacky merchandise of cheap tricksters.

If a wizard really wants to turn a human being into a corpse, all they need to do is pick one of the countless ordinary ways the body can fail and give it a push in the wrong direction. Barring that, incapacitation for long enough for someone to finish them off. That can be done with a bit of severe nausea.

When that first wizard found a way to kill someone with their mind, the methodology of violence began its convergence on the path of least resistance. Death holds out her scales and weighs reliability, cost, speed, difficulty; the fewest words, the simplest actions, the most reliable outcome. Less efficient methods are winnowed out until the state of the art settles in temporary equilibrium with the defenses possible against it. A glance, a hand sign, a murmured word, and then it’s done.

Spell: Kill

Target may save to prevent spell from taking hold; success will still result in an instinctual hind-brain recognition that something is attempting to kill them.

Thankfully, I don’t need to make further special mechanics for this, as Mr. Mann already made Magical Violence (Thank you Luke Gearing for solving rpg combat) and good ol Sanity checks can cover the rest.
 

d12 ways a wizard can turn a human being into a corpse

1. Induce Stroke
2. Supersonic Pebble Acceleration
3. Internal Decapitation
4. Evoke Pancreatic Cancer
5. Aura of Anaphylactic Shock
6. Blood to Ice
7. Intracranial Bleed
8. Seal Trachea
9. Cloud of Hydrosulfuric Acid
10. Testicular / Ovarian Torsion, followed by just stabbing when they’re down

 

d10 reasons why this hasn’t become the norm

1. Wizards that pose a threat to other wizards tend to get killed by other wizards ganging up
2. The spirits responsible for performing magical acts refuse to kill without extremely good reason 
3. True name / direct eye contact / hair / nail clippings / prized possession of target required 
4. Souls provide decent protection against unwilled magical effects on the body 
5. Defensive amulets have kept up with spell developments
6. No one wants to become a case study for divine retribution
7. Beings killed by magic become ghosts and / or demons and they are understandably angry about that
8. Magic is an ecstatic mental state + overactive pattern recognition, you can’t actually do anything with it
9. It’s still cheaper and less time consuming to just hire some thugs
10. High risk of reflective counterspell