Strange case of 了: 他杀了一个人 vs 他杀了那个人
I'm a native Chinese user and I can tell you [very definitively] based on my understanding of my first language that these two sentences are equally illogical (unless one interprets &quot;killed&quot; as &quot;attemped to kill&quot;), but neither of them are grammatically incorrect.
They are both illogical because, &quot;杀了&quot; means &quot;killed&quot; in the context of these two statements. So e.g. the first sentence means, translated very literally (so literally that it would sound unnatural in English), &quot;He killed a person, but (he/she) did not kill (him/her) to death.&quot;, where the words in the brackets are words that are omitted in the Chinese sentence. You should see how (obviously) self-contradictory this sentence is in English - from my perspective the Chinese statement is just as self-contradictory as the English translation of it I gave.
Similarly, the second sentence means &quot;He killed that person, but (he/she) did not kill (him/her) to death.&quot;
The difference of these two sentences (of &quot;那&quot; and &quot;一&quot;) is completely irrelevant to the determination of whether they are grammatical and whether they are logical.
Edit:
As correctly pointed out in koshiyomi's answer, if the verb &quot;杀&quot; is replaced with some other verb, e.g. &quot;吃&quot; for the two sentencces, then there is a quite subtle difference between the two sentences, because:

It is perfectly natural to say a person ate something but didn't finish eating, whereas it is unnatural to say that a person killed someone but didn't kill him/her to death.
&quot;吃了一个苹果&quot; means &quot;ate one apple&quot; where &quot;一&quot; usually (but not always) suggests that one whole apple was eaten, but &quot;那&quot; in &quot;吃了那个苹果&quot; simply specifies which apple was eaten, with less suggestion that the whole apple had been eaten. Both sentences can be commonly understood as suggesting that the whole apple was eaten though (in cases where it has not been clarified whether the whole apple was eaten or not).

N.B. That the sentences are illogical does not mean that they cannot be used to good effect - e.g. often contradictory sentences are utilized in literature to achieve certain effects. For example, in literature you might have something like
&quot;He killed that person.
But no, he did not die.&quot;
where the first sentence describes something that is not true but merely apparently true. I remember that this kind of contradictory decriptions were used quite often in the Japanese visual novel Fate/Stay Night.