How awful was Bellatrix’s descent into madness ?

Bellatrix Lestrange's descent into madness can be attributed to a combination of her inherent unpleasant and bigoted nature, her deep involvement in the Dark Arts, and her fanatical allegiance to Voldemort. The 14 years she spent in Azkaban further exacerbated her mental state. It's worth noting that she never received the Dementor's Kiss, a fate that might have left her as a "vegetable".