God asks for animal sacrifices in the Bible and also, elsewhere, gives people the permission to eat meat when they get tired of eating manna all the time, therefore, His law "Thou shall not kill" cannot apply to animals. (He also says he loves the smell of the burning meat of those sacrificed animals.)
God asked for Abraham to kill his son as a sacrifice, but did not allow him to go through with it in the end, and provided an animal for him to sacrifice instead. But God in the Old Testament is not against the killing of people, those he considers his enemies who won't obey him, and the enemies of his people. This includes the murder of newborn babies.
God is extremely wrathful in the Old Testament especially, and sometimes seems to be downright evil. If you don't believe me, I can quote passages that will leave you questioning the truth about the loving God of Judaism and Christianity.
Now, I am a vegetarian myself, part vegan, and an animal lover. I also study Hermetisism, and have knowledge of many other spiritual and religious belief systems. Personally, I am inclined to believe in what is known as Natural or Universal Law in which you are not allowed to take anything that is not yours to take, first and foremost, another's sentient being's life. It matters not if that life is human, animal, or other.