Bu zehir insanı öldürür.

A word-for-word breakdown is:

• bu = this
• zehir = poison
• insanı = the person / the human / a human being as the direct object
• öldürür = kills

So grammatically it is:

• Bu zehir = subject, this poison
• insanı = object, the human/person
• öldürür = verb, kills

Turkish normally puts the verb at the end, so the sentence order is subject + object + verb.

The -ı on insanı is the accusative ending. It marks the noun as the direct object of the verb.

• insan = person / human
• insanı = the person / the human as the thing affected by the action

Since öldürmek means to kill, the thing being killed is the direct object, so insanı fits naturally.

That is a very common question.

Yes, in many cases the accusative does suggest a definite or identifiable object. But Turkish also uses singular nouns in a generic way, especially in statements about general truths, human nature, or typical effects.

So insanı öldürür can mean something like:

• kills a person
• kills a human being
• kills people
• is deadly to humans

In other words, insanı here can stand for the human being as a type, not necessarily one specific person.

A very similar kind of sentence is:

• Sigara insanı öldürür. = Smoking kills people / kills you / is deadly to humans.

Öldürür is in the aorist, which often corresponds to the English simple present in sentences like this.

The aorist is commonly used for:

• general truths
• habitual actions
• things that usually happen
• characteristic effects

So here öldürür means something like:

• kills
• will kill in the sense of is capable of killing
• is deadly

It is not the same as:

• öldürüyor = is killing right now
• öldürecek = will kill in the future

Yes.

• ölmek = to die
• öldürmek = to kill

Historically and grammatically, öldürmek is built from ölmek with a causative idea: to cause to die.

So:

• ölür = dies
• öldürür = kills

This relationship is very useful to notice, because Turkish often builds verbs this way.

Turkish does not have articles like English a and the.

Instead, ideas like definiteness are shown by things such as:

• context
• case endings
• word order
• whether bir is used

So English may need a, the, or no article at all, even when Turkish uses just one noun form.

That is why insanı can end up being translated in different ways depending on context:

• the person
• a person
• humans
• people

In Turkish, third person singular often has no extra personal ending beyond the tense/aspect marker.

So:

• öldürürüm = I kill
• öldürürsün = you kill
• öldürür = he/she/it kills

That is why öldürür by itself already means it kills here, with bu zehir understood as the subject.

The most neutral word order is the one you see here:

• Bu zehir insanı öldürür.

That is the standard subject + object + verb pattern.

Turkish does allow word order changes for emphasis, but the meaning or focus can shift. For example:

• İnsanı bu zehir öldürür.
  This puts more focus on insanı or contrasts this poison with something else.

• Bu zehir öldürür insanı.
  This sounds more marked, dramatic, or stylistic.

For a learner, the original sentence is the best neutral version.

Not exactly.

• Bu zehir insanı öldürür sounds like a general truth about what this poison does to a human being / humans.
• Bu zehir insan öldürür has a more indefinite, non-specific feel, closer to This poison kills people or This poison is used to kill people depending on context.

The version with insanı is the more natural one for a broad statement about the effect on humans.

That changes the nuance.

• bir insanı = a/one person with more emphasis on one particular person or a single human being
• It sounds less like a broad general truth and more like a statement about one instance, one victim, or one unit of effect.

So:

• insanı = more generic, the human being / humans
• bir insanı = a person / one person

Those two vowels are often tricky for English speakers.

• ı in insanı is the dotless i.
  It is not like English ee. It is a deeper, more central/back vowel with no exact English equivalent.

• ü in öldürür is like German ü or French u.
  A useful trick: try saying ee while rounding your lips.

A rough pronunciation of the whole sentence is:

• bu zehir insanı öldürür
• approximately: boo ze-hir in-sa-nuh öl-dü-rür

That approximation is not perfect, but it helps you get started.