Is this a sign of mental illness or just a strategy on her part ?

My older daughter had a dog that died which is commonplace, trust me. Where she lives the ground is rock and she could not dig a hole to bury it so she burned it. This has me upset since she burned her old boyfriend’s books ten years ago when they broke up. I mean I’m not upset over the books but it just seems very strange and almost demonic to burn a dog . She lives on 3 acres of woods, too. How do you feel about it? No way I could do such a thing nor have I heard of it. Am I over reacting?