Is one able to further distinguish people who killed a human? In English we have the word "killer" to just say that someone killed a human being. This would literally translate into "Töter", wouldn't it? However, as far as I know, this isn't really a commonly used word.
Do I have to stick to "Mörder" (murderer)? The problem is that this means (at least to non-native speakers), that this person was found guilty for murdering. But what about soldiers etc.?
There are several juridical nouns for different kinds of homicides. @KilianFoth mentioned the most important ones in a comment already: Mord ‘murder’, Totschlag ‘man-slaughter’, fahrlässige Tötung ‘involuntary homicide’, Körperverletzung / Gewaltanwendung / unterlassene Hilfeleistung mit Todesfolge. There are also several (related) verbs: ermorden, töten, erschlagen and more specific ones.
Although it would be possible to derive actor nouns from these, e.g. Töter, most are not conventionalized. I think Totschläger in particular used to be more common, but the word is more likely being used to refer to a certain kind of club nowadays. Note that Totschlag does not require beating or hitting, but erschlagen usually does. Totmacher, like tot machen ‘make dead’, is only found in child-like speak.
In legal contexts, Mörder is restricted to intentional killings and some further conditions, otherwise it’s used frequently and without much discrimination. The anglicism Killer (and the verb killen) tends to be limited to the more ruthless cases, as in Auftragskiller ‘hitman’.
There is a number of special nouns by frequency, method or target of killing, but most are just compounds of Mörder: Mehrfach- / Massen- / Serien- / Kinder- / Frauen- / Sex(ual)- / Ritual- / Raub- / Axt- / Giftmörder, but also Würger ‘strangler’ (and erwürgen), Todesschütze ‘shooter’, Attentäter ‘assassin, terrorist’, Messerstecher ‘stabber’, Bombenleger ‘bomberʼ, Schlächter ‘slayer, slaughtererʼ.
A killer who hasn’t been convicted yet is usually called mutmaßlicher Mörder or vermutlicher Mörder or, if one wanted to express some doubt about the verdict, angeblicher Mörder. These adjectives can, of course, also be applied to the more generic Täter or Gewalttäter, which require contextual information about the crime in question.
You may want to note that several verbs relevant in this context require the object Mord (or Totschlag etc.) to be in the genitive case, which is rather unusual, e.g. jemanden des Mordes beschuldigen / verdächtigen and jemanden wegen Mordes anklagen.
There have actually been court cases to determine whether the Tucholsky quote “Soldaten sind Mörder” was a defamation of soldiers. Better avoid. “Soldaten töten” would be much less controversial.
There are few other “jobs” that involve killing of humans. Executioners are properly called Henker or Scharfrichter, but if there still was capital punishment in German speaking countries I guess there would be a more complex, harmless sounding, bureaucratic designation like “ausführender Justizvollzugsbeamter”. Suizidhelfer would be used for assisted suicide, whereas any compounds of Selbstmord are avoided for the most part. The (yellow) press features a whole bunch of (mostly awful) metaphors, e.g. Todesengel or Todesfee ‘angel / fairy of death’ for ICU nurses pulling the plug or overdosing their patients.
PS: Every accident happens for a reason (Unfallursache) and usually someone is therefore responsible for it (Unfallverursacher) and likewise the cause of death (Todesursache) may be the result of the actions of a Todesverursacher. Although an uncommon, perhaps unidiomatic word, this cannot really be considered a neologism.
There is no neutral noun corresponding to killer. In order to convey the same meaning, you’ll need to rephrase the sentence so that you can use a verb instead:
I am no killer!
Ich bringe doch niemanden um!
The reason why there is no German generic term equivalent to the English "killer" is probably - at least partly - an aspect of cultural difference.
In Germany (where penalties are generally significantly less harsh than e.g. in the USA) a person found guilty of negligent homicide (fahrlässige Tötung) would often receive a short sentence usually suspended; the maximum sentence are 5 years and are normally only applied if severe recklessness in the culprit's behaviour was a key factor.
1) A person committing a murder,
2) a person who performed a justified killing (self-defense or a police officer in an emergency situation), and
3) a person making a terrible mistake that tragically kills someone else
are considered so extremely different, that - from a native German's point of view - a request for a generic term for these seems quite absurd.
Instead of Töter (which does exist) you are more likely to find a participle construction Tötender, though not very likely either. And it still conveys a sense of subjective activity.
You'll only find "sterbender", hardly *Sterber, too, although nominalizations on "-er" are productive, e.g. "break-dance-r".
The reason that neither is likely to be found in a context where the difference is paramount is mostly the strong taboo around the topic. People would like to avoid thinking about it at all, and, if pressed to, will present a situation where the killer is not guilty of murder to any degree as if the killer was completely passive and the dier (the deceased, the victim of own making) ran into the knife.
Hence, the expression "in's offene Messer laufen lassen" (to let run into an open knife) is a two edged sword, that mostly denotes Arglist (wanton) or Fahrlässigkeit (neglect of responsibility; cp Gefahr "danger").
Whole books could be written about what's been dubbed double-speak or new-speak (news-speak?), see e.g. neusprech.org. So, common expressions are "Gefahr beseitigen", "ausschalten" or "unschädlich machen". I'm not sure, nor do I really care, what the Nazis used in official speeches, but glorifications like "Retter", "Held", "Richter", "Schicksalsbote" etc. are possible.
If it's a sad accident, then "Verantwortlicher", "Auslöser" and the like are neutral, "Schütze" (note "schützen" - protect), "Jäger", "Polizist" or any other kind of occupation that were factual may be applicable. But a generic term would have to be paraphrased.
