Might even almost be having some PTSD from the minute Cailan started on about getting help from Orlais. Given what he tells you about what he saw done by Orlesians, I do thing there's PTSD that might be a logical trigger. I don't see him as evil at all. I see him as not believing it is a blight which he says in one of the cutscenes and truly believing that then his obligation is to see the Orlesians don't try to retake ferelden.
I know this is probably highly debatable to many but given what he lived through, what he remembers and the fact that he knows the orlesians are coming and he doesn't believe it is a blight AND he is not offered any proof to the contrary by duncan, I really can see his side of things and if I were there watching it from his perspective I would have been very concerned about orlesians coming.
Also, I don't see why he has to keep going after the wardens.... there are only a few. Why would he even bother? They could slur him but who would believe them? That's the part that when I examine all of it, I still don't quite see why he goes after them at all.
Loghain definitely has a huge aversion to the Orlesians in general and is very hair-trigger when it comes to them. His similar mistrust and dogged pursuit of the Wardens I believe stems from a few related things.
1: The Wardens have already been exiled from Ferelden once before, when Sophia Dryden broke their political neutrality and attempted to overthrow King Arland, along with inciting rebellion amongst the nobility. Whether or not he was a tyrant and the Wardens got involved after the rebellion was already starting, no-one really knows, since they ultimately lost. When Loghain mentions the Warden's exile before Ostagar, I always got the feeling that he knew the entire story and that was part of why he didn't trust the Wardens.
2: If the Wardens never intervened during the Orlesian Occupation. If the Wardens were willing to take down tyrants and incite rebellion once, why did they never get involved during the Ferelden Rebellion, when power-mad Chevaliers murdered, raped and pillaged their way across the countryside? Again, this would reinforce a belief that their claims of "neutrality" are suspect and entirely self-serving, designed to allow them access to all governments across Thedas. They get involved when they think they can get something from it.
3: He doesn't believe it's a true Blight, rather the remnants of a dying order trying to gain more power in Ferelden, using an idealistic and foolish young King to do so. This one doesn't really need explaining since Cailan was pretty much ready to follow the Wardens at the drop of a hat, making him the perfect puppet king for the Wardens to manipulate.
4: Orlesian and non-Ferelden Wardens at Ostagar. In addition to the major contingent of Orlesian Wardens, backed by Chevaliers that Cailan wanted to get from Orlais, quite a few of the Origins are non-Fereldans. The Dwarven and the Dalish Wardens can make it pretty clear to him that they don't care about Ferelden or it's King, while DA2 also reveals that the Human Mage is from the Free Marches. Coupled with Duncan being of Rivaini descent, a nationalist like Loghain probably isn't going to be relieved to have all these "foreign" Wardens defending Ferelden. With the Orlesian Wardens and Chevaliers backing them up, Loghain's going to be more concerned of a coup attempt.
I'm in the camp that believes that Loghain always thought Ostagar was going to be the staging ground of a coup. Cailan was planning on selling out Ferelden to foreign invaders, using the talk of the Blight as a smokescreen to get forces in and prepared to annex the country back to Orlais, while he chucked Anora for Celene. And we know from RtO that he was partially right about this.
He just didn't bank on the Blight being real and the Wardens genuinely being there to take on the Darkspawn, rather than being part of a conspiracy that Loghain convinced himself was there. when it became clear that's what it was, he was left blindsided. Continuing to blame the Wardens and hunt them down was simply damage control to prevent anyone realising how badly he's screwed up!
Dead men tell no tales, after all.