Aquinas on God and Evil
Oxford University Press
Aquinas argues that things that have being are good in themselves. Evil is thus a privation of being. Also, for Aquinas, there is a real distinction between essence and existence in creatures. From this, Aquinas is able to put forward an argument as to why there is evil in the world when it is created by God who is all good.
For Aquinas, there cannot be a material world without creatures hurting each other.Better still, God could have created a world without animals killing each other, but it would not be the world we live in.
For Aquinas, it is not sufficient to say that God created us with free will and therefore has nothing to do with our evil actions. Rather, God makes us do what we do by our own undertaking.
This is a highly readable introduction to Aquinas on God and evil.