For those who do work with stingrays, you know that you can be quite a distance away and still be hit. Don't jump to conclusions that he was either messing with it, or didn't know what he was doing. This is a freak thing. It is not easy to be stung by a stingray, let alone right in the chest. From the current descriptions from his crew, a camerman was swimming in front of the ray, and Irwin was snorkeling off too the side of it. But they can react very quickly, and if it felt cornered, then it would react. He may not have been doing anything to the ray at all.
Appreciate that he made a decision to risk his life -indeed sacrifice his life - to bring nature and a passion for animals that are often maligned to the public. He made a great impact, established a genre, and if a single kid (and I am sure there are more) have a love of nature because of it, then I suspect he would think it worthwhile. I think if he had a choice of how to go...something like that would be it. Out with animals, probably killed by one instantly.