Arguably, he is one of the strongest mortals, so that probably play a part. After all, the Demon Hunters followed him, even while they were thinking that he was a madman, was because they considered him the only one capable of defeating the Legion. That is probably their biased opinion, but it should also tell part of his power. Even when he was extremely weakened, he was still put up a good fight against us and Maiev, after all. Not to mention he was the first one who did some real damage on the Legion since eons ago.
And he didn't give Azshara the Demon Soul so she could summon Sargeras in the latest version of that story (Knaak's WoTA triology). He was planning to use that portal to do what happened - banishing all demons back to Twisting Nether - and he let Tyrande + Malfurion knew his intentions before the demons were defeated. A rash action, indeed, but not without good intention.