Grant did drive tiffany to her death, he was abusive to her. On the night tiffany died he was running away with courtney. If he never did that tiffany would be here today.
Tiff wasn't perfect either, lying about grant's attempted murder of her. Not saying Grant isn't responsible but it was a toxic relationship on both sides, Beppe being involved didn't help
Tiffany was a spoiled princess who tried to use Courtney as a pawn every time she didn't get her own way. She was going to have Grant sent away for something he didn't do. Soap justice being what it is, her sing-songing "You know you didn't do it and I know you didn't do it, but no one else ever will" deserved punishing.
It was definitely Grant's fault. He didn't kill Tiffany but he definitely got her killed. Just like Phil hasn't intentionally killed anyone but he's gotten several people killed.
