Kael’thas sins will be laid out in Revendreth, which will probably go into far more depth of why Kael’thas is redeemable than I could go into here, though the short answer is until he finally snapped and went insane after being left for dead in Tempest Keep, Kael’thas had done everything he did for his people. Sylvanas only ever did anything for herself.
I believe Sylvanas deserved to go to the Maw even before Wrath of the Lich King, ever since at least this part from the Arthas novel:
- The human girl was on her knees. Keever stooped down, yanked her head up by her hair, and when she opened her mouth to cry out in pain, he poured a cup of something down her throat and covered her mouth, forcing her to swallow.
Sylvanas watched while she struggled. Beside her, the Forsaken male accepted the cup that Faranell offered without protest, draining it dry.
It happened quickly. The human girl soon stopped struggling, her body tensing, and then going into paroxysms. Keever let her go, watching almost curiously as blood began to stream from her mouth, nose, eyes, and ears. Sylvanas turned her gaze to the Forsaken. He still regarded her steadily, silently. She began to frown.
“Perhaps this is not as effective as your—”
The Forsaken shuddered. He struggled to stand erect for a moment longer, but his rapidly weakening body betrayed him and he stumbled, falling hard. Everyone stepped back. Sylvanas watched raptly, her lips parted in excitement.
“The same strain?” she asked Faranell. The human female whimpered once and then was still, her eyes open. The alchemist nodded happily.
“Indeed it is,” he said. “As you can imagine, we are quite—”
The undead spasmed, his skin breaking open in spots and weeping black ichor, and then he, too, was still.
“—pleased with the results.”
“Indeed,” Sylvanas said. She was hard put to conceal her own elation; “pleased” was a pale word indeed.