I see two aspects to this question.
The first one is how easy (=spell level) it should be to kill/neutralise an antagonist ?
The second one is how each Technique/Form combo should compare to other ?
In Hermetic Project, the Intangible Assassin covers well the first aspect.
To complement that, the cornerstone of how easy it is to kill is a mechanical balance between the effect and the penetration. No matter how artificial it feels, it is important that an effect can instant kill, but with no/low penetration because of its level or can easily bypass defense, but requires multiple casts to take down the target. Anything else (=instant kill with high penetration because low level), will completely undermine the core aspect of the Order: Parma Magica allowed mages to see eye to eye because of a relative feeling of safety.
If a ST introduces means of killing with low level spells/high Penetration, his saga will need to find another justification why the Order exist at all and is not run by a handful of paranoid tyrants.
The other balancing act is that indirect effect require Finesse and should leave a chance to the target to mitigate the damage (dodge/fast cast/ other).
The second aspect also require that some facts are agreed within the troupe.
First one is not to use modern knowledge to justify low level spell with killing power. Like designing a ReAu that prevent oxygen to reach a target (ReAu Base level 3 seems to fit the guideline, YMMV).
It becomes more difficult to argue when somebody propose MuAq 10 (Base 2, R: touch +1, T: Part +2, D: Diam +1) to turn the target blood into water for 2 minutes, killing him in the process.
My approach to adjudicate such spells is to look at the intent of the spell and not the mechanic, then select the appropriate guideline. In this case, turning somebody blood into something that will kill him fits more the general guideline "Change a liquid into a liquid that does +(level) point of damage on contact", setting at +25 or +30 the base level instead of 2.
Issue will arise if a spell was design with a non-lethal intent, then a mage finds a lethal application. Then I will adjudicate that since the lethal effect was not part of the initial spell design, the lethal aspect does not work as expected. Obviously, early discussion during session zero can prevent future frustration and disappointment.
And as ST, you should be transparent about that: just say that within Mythic Europe paradigm AND for game balance, spells work according to such and such guidelines.
Also, mention the symmetrical effect: if the PCs have access to it, NPCs as well. That tends to cool down over-enthusiastic murder-hoboish behaviour.
Finally, regarding PCs killing willy-nilly, don't forget that the Divine might decide to intervene: by killing people, mages prevent - even harden criminal - of any opportunity for redemption...