We adopted a mastiff mix in August, last night, he killed a cat in our back yard. We have a 2 year old, and just not sure that we can trust him, and knowing he has had a taste of blood now, what is he capable of? What do we do?

Mastiff are a guardian breed. They aren't aggressive they are protected. The cat probably did live there ? So the dog was doing his job and protected his place and his people. My dogs eat a raw meat diet. If the taste of blood made them want it more my whole family would be done by now lol. I understand your concern with having a young one. I have two sons myself. The only worry id have with a mastiff is it pushing over or running into my kids. But not any aggression at all. This is just my opinion on the breed in general. Ive never owned a mastiff but ive owned great danes and american bullies (both ha e a bad wrap about being killers) my dogs have never shown aggressive behavior towards anyone unless they approach their yard

The Mastiff was only watching out for its owner. If in doubt look up Cesar Milan or Uncle Maddy's website and to ask them the same question and see how they respond back

I did. She stated that his type typically are hunters, but no he shouldn’t be trusted around my baby. I asked what I should do, her response “it’s up to you, but it’s been more than 30 days, so the shelter won’t take him back”

Your family is his pack. The pack is key to dogs. As long as you and yours treat it well, it shouldn't be a problem.You can't be sure why he killed the cat. Not advocating, but the cat may have attacked, no clue. If you are feeling uncomfortable, contact the shelter.