Had Tom been injected too? Where is the used syringe evidence? Have we been definitively told that that insulin rendered them comatose and/or killed them (or was it smoke inhalation)?
I think Tom's nasty nature is being purposefully bigged up but there's been nothing so far to indicate that he's been other than a loving father, has there? So either a massive row with Kate once they got home from the ceilidh which made him decide to punish her in the worst way possible (by killing the girls too) or he didn't do it.
My money is still on Steve, although not quite sure why he would have tried to kill the whole family?