an aggresive cat

Question
I have a cat that is usually very friendly. My question is, how do I stop him from chasing and killing small wild animals around the yard ?  

Answer
Unfortunately you are trying to fight nature here and there really isn't much I can recommend other than not letting him have free roaming outside. Additionally, the fact that he is doing this is exposing him to all sorts of serious disease and parasites carried by the small animals he is killing. It is a serious risk. There are many concerns to letting him out - the average life span of an indoor only cat is about 18 years. A cat that is out all the time is expected to live only 3 years on average. Yours may be in and out, but when he's out he is exposed to the same dangers as any outdoor cat such as poisonous plants in or around your yard that you may not even know about or pesticides. If other cats happen to wander into your yard he is exposed to fights, or catching a parasite or a disease, including possibly fatal ones. If he roams further than your yard, the types of dangers increase. Indeed the best advice to stop the problem you are inquiring about is keeping him in and that will serve him well healthwise as I have explained. Here is a good article to read about this problem:
http://www.abcbirds.org/cats/
Unfortunately, stopping him if you are continuing to let him roam outside may be an impossible task. And considering the health risks involved, stopping him is imperative - by keeping him in. Or, if yu are willing to make the investment, you can puchase an outdoor enclosure so he can feel like he is out but he is in a safe enclosed space. Please read the article suggested at the link above, and near the very bottom you will see a number of suggestions for Cat Enclosures.