Putting Down Animals?

I'd like to know what you guys think about putting down animals, whether it is ones that attack people or just over crowded shelters having to put down animals for lack of space.

I understand the concept, and I understand there is not much other choice. That being said animals especially dogs have feelings, and thoughts I believe and to me their life is just as important as a human life, I am 100% against this and would love to know what you guys think, and why?