When you right click with McCree, his aiming reticle becomes a large circle. In order to hit with all 6 bullets, your opponent would have to be standing still in "shotgun range" inside that reticle. You would have to be in an extremely bad position to be killed with McCrees right click. That ability is to save himself for when someone gets into his kill range.
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All the support are good. A good Symmetra will wreck the SHIT out of someone, her charge ball can create a zoning effect because it travels slowly. She can set up turrets that sup her damage output and can place a teleporter on long run back maps like Hollywood and R66.
Lucio is probably the best, low skill cap, great reward. Wall riding, which at first seems like a gimmick until you learn the maps.
Zen is still fkn epic, even after his orb nerf last month.
Mercy is good, but in order to play her well, you have to master her, she requires a massive skill cap to keep your team buffed and to whip out the pistol and deal damage when needed.
As a new player, I would stay away from Mercy and Zen, until you learn the game a little better.