We had a rotty/lab mix, a rescue dog.
Jack was a lot of fun most times, but he definitely had a protective/aggressive nature.
We had a neighbor caring for two mentally and phisically handicapped adults. Jack sensed their weakness and it brought out his killer mode. I had to be VERY careful with him.
Eventually he started killing deer in the yard at the cyclic rate. He became too much of a liability to keep.....
Since then, we look for a certain profile in a rescue dog. Smaller herding breed blood (heeler, border collie, cattle dog, mix etc) and female.
Here's a few pics of ol' Jack Boy,