Some questions around a sentence(in the form of present perfect)

Hi teachers!

First of all,the sentence:

<I have killed that man last night.>

Questions and my ideas on this:

I know:it's weird to point out the exact time in this sentence because of the rules of present perfect.

?But I am still confused about the reason why we can't point it out.?

I know:to express the meaning of this sentence, it's more suitable to say that<I killed that man last night.>

By the latter sentence,you could only tell the hearer that an action exists and how it exists (it happened last night).

There is a shortcoming of the latter sentence:You can't express "that man is now a body."

(As grammar books say: present perfect cares about how things happened in the past affect the present time.)

And why I say it's a shortcoming?Well,here just let me set a context:

"Don't you dare! My brother will come to help me!"

"Sorry,I killed that man last night."(express by this way,you cannot emphasizing the fact that the man is dead.)

Now the bad man says the correct sentence.But isn't it better if we change it to:

"Sorry,I have killed that man."(yes that man is dead now I am emphasizing this)

But this sentence cannot point out when the bad man kills the man.

So I just want to say:

"Sorry,I have killed that man last night."

BUT sadly, it's ungrammatical.

Could I ask for explanation about this?

(Unfamiliar with formal writing,very sorry about that)