Addiction and bouts of depression do not necessarily make someone a bad person. As you can see in this case, Payton was, by all accounts, at his core, a hard-woking, genuinely good man on and off the field. Sadly, he apparently battled some personal demons but despite his issues, he still did good things and still treated people with courtesy and respect.
As for addiction itself, when a person becomes addicted to something like painkillers, going without the medicine can cause that person to begin to experience symptoms of withdrawal (such as cravings, chills, pain, tremors, anxiety, depression) thus leading the person to continue to take the medicine just to feel normal. In addition, a person can build up a tolerance to painkillers over time leading that person to require more and more of the medication to experience pain relief and therefore greatly increasing the chance of becoming addicted. It's the proverbial "vicious cycle." Unfortunately, many people who are addicted to painkillers can function normally for the most part and most casual observers and even close friends and family may not be aware of the addiction. I hate hearing all this about Walter Payton so long after his passing, but hopefully it will at least spread awareness and maybe spare someone the fate that befell Oklahoma Sooner LB Austin Box over the summer.
R.I.P. Sweetness. We all remember the good - as it should be.