I think you were wrong to shoot at the deer before looking for horns. Just because someone else was shooting at it does not mean you should not look for horns.
I think that it is kind of a raw deal you got from the DNR. They should be more understanding, but that is not how it works.
I did the same thing fishing one time. We were catching walleyes and I asked someone on the water how big they have to be and they told me 14". So we measured the fish and kept only ones over 14". We got off the water and the DNR said they were watching us with Binoculars and wanted to see our catch. We showed them and they proceded to tell us we had 2 or 3 fish over the size limit. I told them the size limit is 14" right and they said it was 15" not 14".
I told them that it was a honest mistake, that they should have seen us measuring them, and they said we did see that, but the law is the law. That ended up costing us about $250.00.