Hello. I have been doing slow introductions with my new younger rats and my two adult boys. Today I felt confident enough to clean the cage and place them together as a group. I watched for 10 minutes and all was fine everyone was exploring and eating. Well when I came home from grocery shopping I found one of my younger rats dead. He had a huge chunk taken out of his butt and his entire head was mangled like no brain or anything. I am so upset I am crying hysterically and I really need someone to explain to me what I did wrong/what could’ve happened. There was no aggression or fighting but I saw one of my adults bullying the other young rat so I separated the young rat from the big boys. I am so freaking heart broken and traumatized from seeing what I can only describe to you as an absolutely horrifying nightmare. Please help me.