I think Tom’s lying about things or omitting to reveal them. Maybe he did the injections and did himself as a ruse but a lower dose. But I don’t think he meant to kill them. And I don’t think he was involved with the fire. I think the fire is someone else.
I’m not sure if it’ll go this dark, but something made me wonder about Tom & Kate being involved in a paedophile ring, or even a swinger’s group held at their house. That could be a reason why the girls were locked in, and maybe the keys were thrown away with the receipt by mistake as Kate was distracted (a stretch but you never know!)