Is killing a domesticated Duck for foraging on others Property a case of Animal Cruelty?

On May 2nd one of my 10 ducks was found almost dead. the duck was still alive at the time around 7:30 pm as I was going to put them into the night enclosure and I took this little 1 year old duck up into the house to see what caused this. within 10 minutes she died. There was a lot of blood drops on the floor as well yet I could not find any reason for this severe bleeding. I put the lifeless duck in a large cardboard box and was going to bury her in the morning and I would have until my neighbor began yelling and swearing at me about my ducks eating his plants in his yard, I then put 2 and 2 together and questioned him about why one of my ducks is now dead also adding in some insults and in his anger he blurted out "Because I shot it" saying he used a pellet rifle. I called the local police and the chief and another officer showed up within 10 minutes. They asked me questions and then went over to question the neighbor then they returned and wished to see the duck, that night before a predator had gotten into the box and did some bad damage to the ducks body but they wished to look anyway. They told me that the Animal Control Officer would be by within the next four hours to take custody of the ducks body for a necropsy. She never showed up but she did show the next day, May 03, 2016 around 9 am. she asked questions and eventually took the ducks body away. On May 05, 2016 The Animal Control Officer and an MSPCA person showed up at round 9 am and began telling me that the conclusion of the ducks death was not from a pellet Rifle but was due to a predator attack. This was impossible as I looked over the ducks body immediately after it died and there were no wounds at all. The only time a predator was in contact with the duck was AFTER it had passed away. Yet both the neighbor and his wife both admitted to my Father-in Law, and myself that he shot the duck and killed it, and his wife admitted to the Animal Control Officer that they in fact shot at all the ducks but ended up killing one. But yet even with these testimonials the MSPCA and the Local Authorities did nothing to these people who killed my duck for doing what a duck does and that is foraging for food. Now if this guy had called or came over and asked me to remove my ducks from his property and that they did damage I would have no problems either paying for or even planting new ones myself as I am the owner of these birds and responsibility comes with ownership. Yet he decided he had all the right to shoot and kill a defenseless duck and let it suffer for at least 3 hours. So if I get this is it OK to shoot at someone if they are trespassing on my property and kill them or is it only illegal if I hit and kill them. This duck did not deserve to suffer and eventually die but main question is why did the local PD do nothing even though these people admitted to other people they did shoot at all 6 ducks of mine yet killing only 1...and if the MSPCA and Animal Control Officer have it their way they are not even putting the death on these people, I do not even really know if the bird had passed at around 8 pm and as far as I know it could have gone into shock and had not died when i thought and the predator might have done the actual finishing bite but also destroying any actual evidence like the pellet wound or even the actual pellet itself, also proving that the bird may have been still alive at the time of the night predator attack...I do not know for sure