Actually, reading this story left me thinking that perhaps we've been a little unfair to old Stardust.
In the story where we supposed he let thousands die, we don't see Stardust racing towards Earth: we just see him saying "A wholesale murder plot! And they're working fast!" and then arriving too late to save everyone.
Considering that the murderers were already on Earth at the time of their plot, and Stardust was extremely far away on his private star, it could be that he left as soon as he realized what was happening and simply couldn't get there fast enough.
Perhaps his constant frustration over being perpetually out of reach and therefore too late to prevent disasters has driven him to the edge of madness, causing him to overcompensate with the sadistic manner that he avenges those massacres.
On the other hand, you'd think he'd just build a faster way to get to Earth, or move closer, or something. So he's still a massive tool.