Here's the thing I had for introducing the Arrancar Dante. Basically Dante put up an intense fight with Aizen after he saw him slaughter some of the captains. But all that had to be done was make one wrong move and Aizen stabbed Dante through the heart. He realized that Dante had a lot to him and a lot of stuff he hasn't even figured out he had yet, so Aizen decided to make Dante an Arrancar. So basically, Dante kinda had his life taken away from him and replaced with a "new" one.