This question was asked by Corrie.S.P. for Warriors Wednesday.
*Spoilers for the first – (maybe) fourth series and Yellowfang’s secret*
Brokenstar was evil for many reasons.
The first, to punish Yellowfang for having kits and a mate when she was a medicine cat. The rest of her kits died (two she-kits) and only Brokenkit survived. Which, I think is unfair. Did StarClan do that to Leafpool? Or any other medicine cats that had kits and a mate? No. So why did Yellowfang’s have to be the only one to severely punish all of the clans? I don’t know.
StarClan (really the authors) could have thought this through but, I guess, Leafpool did have to take time off from being a medicine cat but, that didn’t effect all the clans, even driving out a clan.
Second, his ambition and the fact that he was nursed by Lizardstripe. Lizardstripe didn’t even want kits! Especially not Brokenkit. He had a horrible mother and no father (that he knew of).
Those are probably the main reasons he is evil, there might be more, but these two seem the most accurate.