He had connections to Ouroboros and possibly started the Hundred Days War by getting jaegers to attack and destroy Hamel to try to annex Liberl? Along with trying to show the empire’s might.
Causing the deaths of many of the series characters family and friends: Agate’s sister Mischa , Estelle’s mother Lena, Joshua’s village and family , Crow’s Grandfather and possibly many more.
Erebonia was at fault ( Trails in the Sky SC ) and he had the connections, motives and underhanded methods to do it. Underhanded methods and ruthless decisions are his way of doing things as shown especially in Cold Steel series.
From what I’ve seen in the the Zero no Kiseki walkthrough here:
He is involved in the ongoing corruption of Crossbell and could be supplying weapons and other things to gain support. Allegedly Rean’s father is well aware of a good bit of info on Ouroboros’s plans.
The chat between Oliver and Osbourne in Trails in the Sky the 3rd gives a nice bit of info.
Quoted from Kiseki wikia:
He side steps Olivert’s probing questions about the neighbouring states Osborne has annexed, which he points out occurred willingly on the part of those states though Olivert notes the rather convenient timing of worsening conditions in those countries that spurned their agreement, and his connection to Ouroboros, which Osborne doesn’t confirm explicitly but clarifies simply that he will use whatever means to bring about his reforms.
Link to video:
I think Osbourne used Duke Cayenne as a tool for the civil war in CS2 as while the eyes were away from Osbourne he was free to plan his entrance, further annexation of places and possibly brought an end to Class VII. Oliver’s project had surpassed his expectations and, like the Bracer in Liberl, could be a possible threat to his plans like how he got rid of the bracers in Erebonia to an extent.
I do wonder if we should think if Osbourne is a bigger threat than Ouroboros? Though then again, Lecther will play a key part in that given which side he is really on.
I hope you enjoyed reading this and thanks for reading.
Related theory about Olivert: