I dont believe GR meant that future humans would be completely free of strife and total BFFs with everyone but that petty fighting and the like would be rarer, the same way humanity (for the most part) is less barbaric than a thousand years ago. ST main characters are largely presented as heroic figures. Heroic characters can be heroic only if they are also capable of evil. If humans can only do good things, they couldnt be called heroes,they'd be little different from machines.
As for Sisko/Picard, of course Sisko doesn't believe Picard is responsible for his wife's death, otherwise he'd go right for Jean-Luc's throat the moment he sees him. In his mind however, Picard is associated with the most painful moment of his life, so meeting him again under any circumstance isn't going to be a happy experience.