Despite the fact that he does so painfully and against his victims will he does grant people immortality. He does not approve of Thresh's incarceration and torment of the dead. And while he lived he comforted those who lost people they cared for.
Judging by his updated lore, he truly believes he's doing people a great favor by granting them eternal undeath. So his "heart" is in the right place, I guess..?
Overall Karthus belives that what he does is good,but the problem is that Karthus forces his belief onto others. It is also rather understandable,because death is a very intimidating thing,so he probably thinks that people will understand the benefits of undeath once they are dead(also he probably wants to kill people before guys like Thresh get their hands on them).
The problem is that even in death,not everyone is ok with being undead,because when Illaoi purified the spirits from the shadow isles,some spirits where happy to go on to the afterlife,while others fled to remain undead.
Karthus is okay, but he forces people to become undead. He's zealous and doesn't understand the beauty of life.
Karthus should really be called "The Missionary of Death" but i feel like that would lead to some real hate from certain populations.
Karthus has the classic villain quality of hubris. He hasn't even considered that he might be wrong.
It's tragic really.
