Yakov has not been that bothered about the tournament, because competition is not really part of his make-up, but he was dragged along to the task anyway. He knew something was wrong when Krum attacked Fleur and Cedric, though, because that's just really out of character. He, along with a few other Durmstrangians, I imagine, would have tried to ask for the tournament to be stopped because something must be wrong for that to happen. Obviously, this was not granted (because once the task is on, the task is on, dude). He would have been worried through the whole thing after that and then when Cedric was found to be dead he'd have felt kind of sick.