Sacrifice

Most people eat animals. In fact all of nature is about eating. If you look outside, all you see is one thing eating another thing, from the birds to the insects to the mice to the cats to the fish and everything else. Lots and lots of eating!

It is really hard to comment on the will of animals. Obviously animals have a survival instinct which makes them dislike being killed. However, most farm animals would not exist at all if people hadn't taken the effort to breed them and raise them.

What can be said is that it is allowed for people to eat animals as long as they are treated and slaughtered in the right way, although some people may choose not to.

The "sacrifice" here is about sacrificing the time, money, and effort it takes to raise an animal (or, if it is purchased, to pay for that) and then giving away a good amount of it. In times where people had very limited food options, or in places where many people don't have access to healthy food, this is a big service to other people. It is one thing to advocate being a vegetarian in a place with many food options and where you can eat a healthy diet without meat or animal products, and a different thing when not eating animals can lead to malnutrition.

(This is as opposed to some older religious traditions where sacrificing an animal for deities or other beings was said to offer things that were useful for them such as blood; the Quran clarifies that God, as pure divinity, does not receive the meat or blood but rather acknowledges the sense of sacrifice.)

Sometimes these questions come up because some people look down on Muslims for being barbaric. However I would point out that in the United States, almost everyone considers it traditional to eat a turkey on Thanksgiving, and eating it requires slaughtering it, and sometimes homeless shelters also serve turkey to those in need, yet it is rare to hear people comment on that or treat it as backwards. 

Many animals eat other animals, and many human societies have also subsisted on animals, especially in areas with limited agriculture. Animal sacrifice is not killing for the sake of killing, as one should respect the animal, but killing so that those who are in need can have food. 

Plants are also living creatures and also belong to Allah! Many people interpret the Qur'an as indicating that all beings, whether animal, plant, or stone, have some level of awareness and their own means of glorifying Allah.