Aizen used to be one of the captains in Soul Society until he became a traitor and the main villains for the series. He has a massive number of henchmen demons who work for him, many of who are incredibly powerful and he’s the strongest probably of them all.
One great example of how ruthless Aizen can be is when he betrays his loyal subordinate named Momo by stabbing her in the torso. It’s probably one of the most ruthless things I’ve ever seen a villain do in an anime, even though he fortunately doesn’t manage to kill her. He has a massive amount of strength to not only keep his henchmen in line but also give Soul Society a large challenge, clearly somebody not to be messed with.