Give "Gifts" to You
There are a few hypotheses offered by behaviorists as to why your cat is so determined to leave you their kills. Even though this behavior could be considered disgusting, your cat is accepting you into his group and sharing his hunting expertise with you. He could want to express his gratitude for your care of him, or he might want you to pay him more focus when he brings you mice, birds, or bugs.
Put a bell on his collar to make it harder for him to hunt if this is a behavior you'd like to stop in your outdoor cat.