It seems like a very popular theory and headcanon that Severus killed his father upon joining the death eaters as a way to prove himself or something like that.
But i don't think this could be true. In Spinner's end chapter, Bellatrix accuses Snape of always slythering out of action. He was more of a spy for Voldemort and perhaps didn't do the dirty jobs.
And Severus was also unhappy with the idea of killing Dumbledore. Snape asked what about his soul. This implies his soul is undamaged and he possibly hasn't murdered anyone. (It's possible to kill someone without breaking your soul. As long as it is not cold blooded, remorseless murder. Albus implied Severus would be fine).
Of course he could have murdered before, as we know extreme regret/remorse can heal a soul and/or result in death.
Anyways. We don't know enough about Tobias to know if he had been killes by Severus or death eaters. For now, i subscribe to the headcanon that he died from some muggle illness due to smoking.
