Aha! You’ve solved the season 1 mystery of Field Trip Files!

M. Vicié had promised the Historian that the castle would be eventually turned into a museum, but had since fallen in love with the Gardener, and went back on his word. Instead of giving the Groundskeeper his pension to retire, Vicié demanded that he first build an impossibly large fountain at the Gardener’s request, driving the Groundskeeper mad with rage! So was it the Groundskeeper? Well, not by himself...

When the seemingly quiet and unassuming Historian found out that M. Vicie wanted to change the fate of the castle, he teamed up with the Groundskeeper to plot M. Vicié’s early demise. While the Historian hid under the table, the Groundskeeper cut the lights at dinner. The Historian stabbed M. Vicié and then fled the scene only to return moments later disguised as… the policeman who arrested Mr. Fink! 

Switching back and forth from kind old historian to concerned policeman throughout the season should have been the perfect crime, if not for those meddling kids and that talking d– wait, wrong show. It was the Historian and the Groundskeeper in the dining room with the knife!