An immense amount has already been written about human ethical treatment of animals. There is little point adding to this other than to point out some of the sources of conflict. The main problem appears to be that animals are living responsive creatures and thus are capable of triggering off a whole range of reactions in us, from love to the need to dominate. We are poorly placed to be objective about them. Of course, the same applies to our reactions to other humans but there are widely agreed standards of treatment and people are capable of stating their case in disputes.
Animals fit in different niches in our mental worlds.
By defining an animal appropriately, it is possible to justify almost any sort of treatment of it and equally to justify almost any sort of action on its behalf. A related problem is that our bases for assigning value to sentient creatures vary. At times, intelligence, the capacity to feel pain, religious value, scarcity and evolutionary closeness to Man have been proposed as measures for determining how we treat an animal species. Most of these fail dismally if only because they are so utterly unquantifiable and arbitrary. It seems that our treatment of animals says more about ourselves than about them.