My 7 year old son just killed our pet puppy. Should I be concerned?

This isn't his first pet (he killed his friend's cats when he was 5, we thought it was his terrible twos a bit late and dismissed it) but it is his first dog. The dog went missing about 2 weeks ago and he said he saw our pup run out of the gate. Yesterday night I noticed a really rancid smell coming from his room. I went through his stuff to find the source and found the head of our puppy in his backpack and the puppy's paw in his underwear drawer. We sat him down to talk about punishment last night and he seemed so calm. he seemed remorseless about it. My hubby and I are kind of freaked out. he's had behavioral issues before- punched kids at school, beaten his sister up a few times, gotten in lots of trouble at school) but he's just a rambunctious kid. My mother says I should put him in counseling. i want this behvior to be nickled in the buck, but I don't want to traumatise my son bu puting him in therapy. Should we be concerned or is this just a rebellious thing? What should we do?