The Kingdom of Lordaeron was decrepit and dying by the time Arthas became King and had long since purged most of the human regions. Enter: Othmar Garithos, leader of the human remnants of Lordaeron.
First, some backstory that most of us will have missed due to dismissing him as a textbook racist:
Othmar Garithos was the only son of a baron who ruled the lands bordering Quel’Thalas. After growing up in the town of Blackwood, Garithos joined Lordaeron’s army as a knight during the Second War. While he was fighting to defend Quel’Thalas, a small band of orcs broke off from the main invading force and put Blackwood to the torch, killing all of its inhabitants. Othmar’s family perished trying to defend the lives and homes of their subjects. Garithos blamed the elves for the loss of his home and family, believing that they had diverted troops that could have been used to defend humanity. After the war, Garithos inherited his father’s title and continued his service in Lordaeron’s military.
https://wow.gamepedia.com/Othmar_Garithos
Given such details, his dislike of non-humans, particularly those of the high elf/blood elf descendants becomes much clearer. Quel’thalas had all but stood by in the Second War until the Old Horde was literally on their doorstep. That afterwards, King Anasterian pulled out entirely of the Alliance would have only been adding insult to injury. His anger seems very justified, in that context.
Fast forward to the Third War, and the Cult of the Damned had infiltrators throughout the Alliance. No doubt, some would still have remained within the remnants to provide more intel to the Scourge. Faced with such deception, a ruthless approach to traitors must be taken. Kael’thas working with the Naga, was certain a deception and he was already under suspicion by being a blood elf.
Garithos believed that the Naga were vile and inhuman, and was proven conclusively correct on this. Albeit right for the wrong reasons, as he could not have known the Naga’s long and twisted history.
The man’s greatest ‘crime’ if any, would be that he believed Sylvanas could be trusted. And resulted in his untimely demise.
So today, I call on people to remember one of the greatest heroes of the Eastern Kingdoms - who never ran away from combat, regardless of what his detractors might have said.