I believe their was a combination of things that lead to Matthew Shepard's death. McKinney and Shepard may have been dealing with depression and were using Meth to cope with it. Meth can make a person do terrible things and takes away their self control. If McKinney and Henderson were tweaking they wouldn't have thought twice about hurting Matthew until he was practically dead. I am still unsure if I believe that McKinney and Shepard knew each other. If they were both in the drug circle, they could have easily crossed paths. I don't believe they were working together. I don't think Shepard was a dealer but McKinney definitely could have been. With such a small town they probably did meet. It is possible Shepard owed his two attackers money. It's said money meant nothing to him so maybe not.
There is definitely an issue of classism in the case. McKinney and Henderson both come from poorer families and didn't continue their education. Matthew Shepard comes from money and has a great supportive family. The idea of robbing Shepard because he was an easy target would be completely plausible but because they beat him with such intensity and torturously it wasn't the only factor.
Depression, hate, homosexuallity, Meth, money and classism all seem to be a part of this case. I could never say one factor was the real reason Matthew Shepard was killed. It has to be a combination of all or most of them.