Yaru?

I have seen the yaru word after the -te form of a verb msny times. For example "koroshite yaru".
What is it exactly?
Is it ageru in a more plain form?

offensive approach of "do"(something)

Is it ageru in a more plain form?

Yes, it is ageru in a more casul form to violent form, depending on the context.

Examples:

Bouya, hon o yonde yaru yo:
Hey son, let me read you a story.

Koroshite yaru:
I'll kill you, you S.O.B!

Regarding your example, the verb "korosu" itself has offensive meaning. So, as long as its Object is the person who is spoken to, replacing "yaru" by another word does not make the phrase more polite practically.

The words "yaru" and �gageru" can work both as an auxiliary verb (as in your example) and as a verb.

The word "yaru" implies giving from the upper to the lower.
It sounds discourteous if you say "tetsudatte yaru" to your teacher. Here the verb "tetsudau" means helping (somebody) to do (something).
A man may say to his grandson "Kono pen o kimi ni yaru yo" (meaning "I give this pen to you"), but it sounds awkward if a little boy says so to his grandfather.

When you are asked in a Japanese language test which one is more polite, you should answer "ageru" is more polite.
However, personally I feel "ageru" is often not enough to show politeness.

The word "ageru" literally means making the Object to be at a higher level, and it can be translated as "raise" or "improve" in English. But, "ageru" is often used with only a little or almost no politeness. Many people say "esa o ageru" to describe giving food to their pet animals. Even kids under the elementary school age can say "-te ageru" in daily conversations.

So, I choose another phrase like "-sasete itadaku" when I speak of or to someone at a higher position and hope to show politeness or respect concerning my deed.
(Phrasal) Verb, (1) To a sibling, (2) To a friend, (3) To a colleague, (4) To a boss.
Kaku ,(1) kaite yaru, (2) kaite ageru, (3) kakasete morau, (4) kakasete itadaku.
Kuru, (1) kite yaru, (2) kite ageru, (3) kosasete morau, (4) kosasete itadaku.
Sewa o suru (1) sewa o shite yaru, (2) sewa o shite ageru, (3) sewa o sasete morau, (4) osewa o sasete itadaku.

An English grammatical error is found.
NOT: making the Object to be at a higher level
BUT: making the Object be at a higher level

Sometimes "yaru" is used instead of another verb or verbal expression which is obvious, while "ageru" does not work this sway.
A: Dare ka kono kozutsumi o yuubinkyoku de dashite kuremasenka?
(= Could anyone ship this parcel from the post office?)
B: Boku ga yarimasu.
(= I will.)
Here B does not say "Boku ga agemasu" because "ageru" does not refer to the verbal expression used just before it.

Thank you really much, really helpful!