It was actually the post sting 'treatment' which killed him.
Those things are pretty mean though. They grow bigger than a man, twice as heavy and have a stinger that causes local trauma, pain, swelling, muscle cramps from the venom and bacterial infection. No doubt something that could take down Steve Irwin would have little trouble with less worthy specimens.
The people I know who've been hit by even a baby one don't seem all that keen on a second episode.