Muslims killing innocent Animals?

First of all, there is a difference between killing and sacrificing. 

Let's imagine every word of these two phrases:-
1) At night you and a stranger passing each other on road. When he comes near you, he puts his hand in his back pocket and takes out a knife, catches your throat and stabbing you as much as he can till your last breath.
And you die.
This is KILLING.

2) You have a serious problem and a person comes to you and says " friend I know you are going to judge me as a fool. But yesterday, I had a dream. An angel came to me told me that if I sacrifice myself then not just this but all of your problems will be fetch up." Then his body is getting invisible as cotton goes in the wind. And you realize that it was not his body, it was his soul. He already sacrifices himself for you. 
This is SACRIFICE. 

BUT WHY THEY DO SACRIFICE? and why ANIMALS?

Well, if they do not sacrifice an animal, then they have to sacrifice their son. Yes, it's true. I am not kidding you. If you don't believe it, here is the proof. 

You must have heard about Prophet Muhammad(PBUH). Similarly, there had been many Prophets before Prop. Muhammad.
One of them was Prophet Ibrahim (PBUH), 
He was having trouble sleeping for many days because of a dream. Then he decides to share that dream with his wife Hajar. He said to her " from past many days he had been dreaming that Allah told him to sacrifice his son Ismail (PBUH)".

His wife was silent for movement then she agreed and told him to obey what Allah commanded him. Prophet Ibrahim also asked his son and he was ready for the sacrifice.
The Next Day, Prophet Ibrahim goes to a holy place with his son. He told his son to lay down here, he tied his hand and feet. And he tied his own eyes with a black cloth ( that he can't see e the blood coming out from his child's body). And in the name of Allah, he started sacrificing his son and he was feeling all the blood on his hand, and from under that black cloth, his tears were coming out. When he realizes that his son stops moving his body, he removes that black cloth from his eyes. He can't believe from his eyes that at the place of his son was a goat. And beside him, his son was standing there alive.
Then an angel appears and told him that Allah was testing his belief. And Allah is happy that you are his true believer.

From that day, it becomes a custom and people started sacrificing animals.

AnimalS? What type of? 

Muslim sacrifice Goat, Camel and yes Beef too. 
If you are Hindu, Buddhist or Jain then it might disappoint you because Cow is a sacred animal in Hinduism and other regions are vegetarian even Jain believe that killing even a small ant is a sin.

But, we have to understand that DIFFERENT PEOPLE HAVE DIFFERENT DEVELOPMENT GOALS AND DIFFERENT IDEOLOGY.
So, religion does too.
And not just Muslims, Christians and Jews have Beef too. So, we shouldn't just judge Muslims alone.

ONLY MUSLIMS DO SACRIFICE?

No. Sacrificing is a traditional custom that coming up from ancient times.
Even Harappan and Mohenjodaro people do follow the tradition of sacrifice.

Evidence has been found that Harappans sacrifice Buffalo, while Archaeologists have assumed that Mohenjodaro people had sacrificed humans.

At present day, we can see that not just Muslims but Hindus sacrifice animals too.
If you ever heard or read about 
GADHIMAI HINDU FESTIVAL you know what I mean.

(World largest no. of Animal are sacrificed here)

Thank you,
For your precious time and effort to read.

Kindly share your views in the comments.