Your son does not "need" to learn to kill animals. If you will look into the background of most serial killer of humans you will find that they almost all began by killing and torturing small animals. Learning to ignore their cries of pain and suffering is the first step in learning to be indifferent to the suffering of people as well.
Teaching a child not to step on a caterpillar is as valuable to the child as it is to the caterpillar.
Love your child. Teach respect for life!