I think the whole "Illidan blew up a planet" thing is being a little bit misrepresented here. He basically did the same thing ner'zhul did, except he did it intentionally and he is a stronger sorcerer than ner'zhul. He also had the benefit of being able to 'improve' on ner'zhuls work and in the book he didn't even know if it would destroy the planet he just figured it was worth a shot.