We hear his motivations (wanting to save Fereldan, paranoia about the Orlesians) but some of his decisions seem to fall outside that scope. Poisoning Arl Eamon? Contracting a blood mage to do it? Allowing Arl Howe to run roughshod over the people? Was he just a really bad when it came to choosing allies?
He is not inherently evil, but by the time of Blight, he becomes an evil-aligned person.
That said, his fall is not inevitable. If his son-in-law and daughter are not such a pair of useless young fools, if some high nobles like Eamon are not so power hungry and malicious, if the warden commander of Ferelden is not such a reckless and irresponsible opportunist, maybe Loghain will not have to sink that far.
Loghain was not inherently evil. He was damaged, a soldier willing to get his hands dirty for the good of his king and nation, a veteran suffering a long-term case of PTSD, and he was a terrible politician. He had outlived his best friends, his main enemies, and he unfortunately lived to become very nearly the thing he initially hated - a bully and a tyrant - but there was no malice in his decisions or actions.