Hiya!
All that goes completely against:
The rules as written.
The rules as intended.
The rules as fun.
Against RAW? Why?
Against RAI? ...I guess this is "open for interpretation", because I see Time Stop as the ultimate magic (it's 9th level), and if you have 'control of time'...well, I'd guess the intent of a 9th level spell that lets you stop time means you can use it for something other than playing a practical joke or two...
Against RAF? Depends on your definition of "a fun time was had by all". If this would upset your table...then house rule you can't do this (call it a "paradox" type thing maybe...the target was destined to die some other way, so no matter what the wizard does...like two daggers in the skull...
something will let the victim survive or 'come back'). If your table has no problem with this, go for it. Fun is HIGHLY subjective, after all.
As for those saying "No, you can't just 'insta-kill' someone with a 9th level spell!"... do you let paralyzed people get 'insta-killed'? I mean, someone is paralyzed by some ability or poison...and a player says "I walk up to him and chop his head off with my axe"...do you just say "no, doesn't happen"? Or do you say "Ok, you kill him"? I'd bet dimes to dollars most DM's wouldn't be able to justify why a completely helpless individual can't have his head cut off. If the DM says "No, just crit damage", you instantly set your campaign up for "comical fantasy".
King: "Off with his head!"
Executioner: *SHWAAAK*
PC Prisoner: "Ow! Stop it!" (takes 36 damage)
Exectutioner: *SHWAAAAAAK*
PC: "Hey! Cut it out! ...hahaha...sorry, no pun intended, really!" (takes another 36 damage)
King: "OFF WITH IT!"
Exectutioner: *SHWAAAK SHAWAAAAK SWAHAAAAKKKK!"
PC: [I use my Lay on Hands and heal for 75 points] "Look. This isn't going to work out. The king is evil and you know it...you should really..."
Exectutioner: *SHAWWWWK* [36 damage]
PC: "Seriously?! Knock it off, will ya! You already cut my hair and got blood all over my clothes"
Executioner: *SHWAK*
PC: *sigh* "Keep going...another...what, four of those and you might kill me...finally. Won't matter though. Once I get down to 0 I can pop right back up again...you know that, right?"
...hehe...

So, yeah...just ruling "No, you can't kill someone by plunging daggers through their eyes and into their brains while they are time stopped"....that would be just outright "weird" for a ruling, imho.
I still don't see how the rules don't support this action...
^_^
Paul L. Ming