because negative health is a good thing if you know what you're doing, can make you near impossible to beat unless the other person know specifically how to deal with you.
Their isn't a hex code for negative life, rather, th achieve a negative health in D2, you must put the health extremely high, it "rolls over" or whatever, and becomes negative.
I got off open bnet a while ago and don't rememebr too much, a site I think could help you would be:
http://www.nightshiver.com/ericjwin
read his tutorials, you'll learn something, thats all I can think of.