Ingells discovered the truth, and knowing that Pontiff Court will deem him and his men as heretics (traitors), he decided to take potion made out of bloody shade. As seen in Ortel Castle battles, it turned them into bloodlusted mutants - fomors so to say from season 1 propaganda. The "not even if you kill us all" refers to the prophecy, which says that the Goddess will return once every single fomor has been slain. This is not true, since the prophecy is made up, therefore the quote.
We were suspected to be traitors as well due to helping Ingkells in recent past, not that Riordian liked him or us anyway. This is one of the reasons why we decided to take care of this case.
As side note: season 2 is parallel/alternate timeline, where we avoided season 1 events so they don't happen. Well, what was meant to happen did anyway - including Ingkells thing. Except, this time one-man-army (we, the mercenary) didn't clean up this mess, so Royal Army and the Crimson Blade Mercenaries had to raid Ortel Castle. This is seen in no other than the Requiem of the Lost One - battle against Zecallion, one of Ingkells subordinates.
Back in time when Aodhan was serving in the Royal Army under Ingkells' command, his family has been killed by Fomors. Due to late Season 3 events, we are asking humans and fomors to unit against a common enemy. Aodhan doesn't like that, and in result opposes us.