I can't imagine Snape as simply ESE it just...doesn't fit with all that's going on, imo. And you're right--I don't want to start assigning murders to him when there's not a single person that we know of that he'd killed before Dumbledore. I suspect if there was someone he'd killed personally we'd know about it. It's a big deal in this universe.

He has killed DD, though I think we don't know the whole story there. He was able to do it, but I think he had a reason to do so that will be important.