You are all right and after EXTENSIVE research, as there aren't many %health damage over time abilities in HotS yet, and a lot of tweaking I think I have made Death and Decay a useable skill.

I really appreciate that Deloctyte, even the critiques :). Some of them are a little tongue in cheek on purpose because KT was always a little snarky lol. Figuring numbers by the way was a pain it was a whole lot of playing Kael'Thas, and Jaina in test matches and figuring out Arthas, Diablo, ETC, Kerrigan, Falstad and kill times, and how much their health grew. I tried to balance him a lot by playing Zagara and seeing her numbers as well. Overall I was never one to leave a job half done :P

Mana Detonation is actually one of Kel'Thuzad's abilities in the Naxxramas Raid, and although I agree with you on Death and Decay as it was one of my favorite abilities in Warcraft 3. I was having a hard time balancing it when I brought the damage down it didn't feel like a heroic, when I kept it as is you're right it felt far too over powered. When I made it only affect structures, KT lost all his team fight application, when I made it affect only heroes, minions and mercenaries he felt...

Very complex I like the skill cap and the upgrade paths. Seems like quite a devastating assassin!