Why were animals killed in the OT? For many reasons Jews had to offer an animal for healing their relations with God. But why needed those animals to be killed?
@j.atkinson could not answer due to account limitations. I am a minister of a church he attends and he asked me to answer for him using some of our material.
It seems as if those who answered the questions on the other threads have different answers than the one I am giving.

During a year, any Israelite was able to bring a sacrifice as a sin offering. This could be for himself alone or for his family. There was the Day of Atonement where two goats were chosen for the entire nation. The KJV is filled with mistranslations and is not a good rendering of this. We read through correct translation using the Hebrew that 'One goat as a sacrifice on behalf of the people and one goat for Azazel." 

Now you may be thinking, I HAVE NEVER HEARD OF Azazel before… what is that?

I open up Leviticus chapter 16 to find verse 8 at the footnotes of the page mentions Azazel.
To understand this we need to refer to some books some people do not commonly associate with Scripture. The Book of Enoch talk of 1/3 of the angels who rebelled and who were cast down to earth for their rebellion. The first 6 chapters of Enoch tell of this conspiracy of the angels. It gives names of about 20 main culprits. One of the rebellious angels is named Azazel. Remember how Scripture records of angels who left their first estate?

Moving on, we learn that Azazel taught the people on earth many wicked things. Azazel is said to have taught the people the art of warfare to fight wars for the purpose of conquest in pursuit of looting and such.

The earth was left in a wicked condition because of these angels teachings. Enoch tells of the people dying in great numbers and their bitter cries of complaint going up to heaven. God sent a committee of four archangels to investigate the matter and to report to him how bad the situation was. 

Enoch records the angels investigators looked the situation over to make a report to Yahweh. These angels reported, "Thou seest what Azazel hath done, teaching all wickedness in the earth". Yahweh said, "Therefore unto Azazel ascribe all sin".

Individuals do sin but this is said to be in large part because of Azazel teaching people to sin. Yahweh tells one of the archangels "You take a chain and tie up Azazel and cast him into a pit out in the desert. Let him stay there and think it over for awhile, until I get around to his final judgment." Now we know who Azazel is.

Two goats are selected, one as a sacrifice on behalf of the people of Israel and one for Azazel.

The message sent to Azazel by a goat is, "Look Azazel, all these sins are yours, and for you there is no sacrifice. You must pay for all of these sins". This is indeed fair enough; otherwise there wouldn't be any justice. This is all done on the tenth day of the autumn month of Tishri.

This is the source from a church affiliate though we hold publishing rights of all Comparet's Works:
https://israelect.com/ChildrenOfYahweh/Comparet/great_jubilee.htm