-Garrosh and Arthas went down the path they did because they had good intentions originally, even then they still had some redeeming qualities
(so the posibilty of redemption isnt entirely impossible)
-Sylvanas’ motivations are unclear even now but they seem to entirely selfish and far more damaging that anything Arthas or Garrosh ever did
-With Arthas at least there was a degree of separation from us due to the rts nature of the game
-Due to the mmorpg nature of the game it can feel for some people as though everything Sylvanas is doing she is doing to us
-Arthas and Garrosh are simply cooler 
I can blame both. Its not like Sylvanas wasnt already shaddy as hell before this.
https://us.forums.blizzard.com/en/wow/t/sylvanas-is-a-raid-boss/874653/19
Weapons and gear dont carry over to the shadowlands as per Alexandros, so no frostmourne or helm. Although he himself would probably prefer Revendreth i can see the Jailer sending him to Maldraxxus since how much control Arthas had over his actions is… complicated
Bolvar lost his cool pretty much the second Tirion put the helm on him, and the first lines we hear in the undead campaign is that the first soul Frostmourne claimed was Arthas’ own- which explains why he did a complete 180 seemingly overnight
For that matter, the last lines of the human campaign is that he was driven mad by the whispers of Frostmourne