i’ve been thinking a lot about snape’s parents. where they went, what happened to them. their house was severus’ alone by the end of the first wizarding war. it’s easy to say moved away, but i think if they’d just left, snape wouldn’t have stayed there. or that they died, but it’s hard to imagine that without involvement in the war they both died before snape was in his mid-twenties with no mention as to how this affected him.
( i’m not going to say “if they’d been important we’d know what happened to them” because that’s not how jkr’s writing works. for example: i’d argue remus’ life story is important and yet 90% of the information we have about him comes from pottermore, not the books. )
so... i think snape killed or was complicit in the death of his father, after which his mother was driven out of cokeworth. there is no evidence either way as to how deeply involved with killings and torture snape got, but as he made his way towards voldemort’s inner circle and eventually became one of his most trusted followers, it seems unlikely that he was uninvolved. he was, as we know, willing to see james and harry dead, just as long as lily was left unharmed.
so why wouldn’t tobias snape be dead? why wouldn’t snape have done it? tobias snape was an abusive father, abusive husband and a muggle to boot. those are three reasons a young death eater has to murder a man, with the first two being motivation and the latter simply being a good excuse. at the very least, tobias snape was handed over to the death eaters.
it was the aftermath of this that left snape haunting his parents’ empty house for the rest of his life.