kazami yuuka, shiki eiki, and onozuka komachi (touhou) drawn by takatora

Pax_Cruento said:
that "Namagoroshijanai desukaa" is really hard to be certain of. it's like 3 layers of uncertainty or something.
Namagoroshi = a state of uncertainty
namagoroshijanai = not a state of uncertainty
namagoroshijanai desuka = probably not a state of uncertainty?

"Namagoroshi" (生殺し)　does not mean "a state of uncertainty". You're likely referring to an JP/EN dictionary with incomplete definitions that can be easily taken out of context.

The word is derived from 蛇の生殺し, "the half-killed state of a snake" (or more literally, "the state of being killed while still alive of a snake"), referring to a gravely-wounded. suffering (the "suffering" part being important) snake on the verge　of death after being hit with multiple blows that failed to kill it (snakes being hard to kill [cleanly] due to their agility and body structure.)

So, when used literally, someone being subjected to a 生殺し treatment basically gets inflicted with numerous grievous wounds like the etymological snake, all the while suffering through a state of extreme pain and agony as their life ebbs away. This may be done on purpose (e.g. torture, an overly-skilled opponent playing around, etc.) or by accident (inexperienced fighter or executioner, botched up medical treatment, etc.)

You're more likely to encounter this word being used figuratively though. Here, it refers to the "mental anguish one feels over something left unsettled (without a definite conclusion)". This usage of the word can usually be translated as "left hanging", "stuck in limbo",　"keep (someone) in suspense/trepidation",　or something along these lines. There isn't exactly a one-size-fit-all translation here so you'll have to pick something to fit the context.

Now, "nai" acts as a grammatical negator, making it similar to English "no" and "not" (and their associated contractions). But "nai" (especially when used together with "ja") can also be used to form tag questions. Compare this with English "Isn't this...", "Aren't you...", "Don't they...", which also include negators when in question form.

Unlike English, there is no change in sentence structure, so this (a sentence ending with a negator) can sometimes be hard to tell when in written form (spoken makes its obvious by a rising 'intonation' at the end), so you have to depend on context.

But Komachi's line also has a "ka" at the end, which makes it obvious. "Ka" here is an interrogative (question) particle, making the sentence unambiguously a question.

Note also that, like in English, one doesn't necessarily ask a question expecting an answer. It can also be used to express incredulity, or to soften one's statement (again, depending on intonation, in both languages).

So, to sum it all up:

生殺しじゃないですかぁ
→ Weren't you leaving me hanging there?