Dallas Buyers Club is built on the unusual premise of an apparently bad guy being seduced by circumstances into acting like a good guy whether his heart is in it or not.
Matthew McConaughey's motives, except for wanting to be cured of AIDS, are so little emphasized that about all we know about him is what he actually did, regardless why he did it. And to some extent, we don't even know what he did-- for one thing, we don't know whether he was some sort of unrecognized chemical genius, or whether he was just willing to take advantage of what was permissible overseas but illegal in the U.S., or whether he was just lucky in following foreign folk wisdom and fads.
But whatever he does, it works better than what real doctors like Jennifer Garner are doing, at least for a while, and since he isn't a rich man, he forms the Dallas Buyers Club to help him with expenses and to evade the law. And then things sort of get out of hand and he finds himself responsible for more people than maybe he would prefer to be responsible for.
McConaughey is fascinating to watch, if not 100 percent believable. He lost 50 pounds in order to look like an AIDS victim, and he looks so terrible that it isn't easy to see him as a continuing rodeo bull rider or a street fighter who can take on formidable opponents. But he avoids sentimentality and transformation into a standard hero, and his acting is Oscar-worthy. So is Jared Leto's, as a character of such sexual ambiguity that I had to wait for a pronoun to classify him as to original sex, and you can take it from there.
Dallas Buyers Club is first-rate in more ways than I have time for, so I'll leave it at that.