Some of the most prominent "bad" Transformers include:
* Megatron: The main antagonist in the original Transformers series and many subsequent iterations. He leads the Decepticons in their quest for power and domination.
* Starscream: Megatron's second-in-command, often scheming to overthrow him and take control of the Decepticons.
* Soundwave: Megatron's loyal and silent communications officer, known for his deadly cassettes.
* Shockwave: A brilliant but emotionless Decepticon scientist, often seeking to achieve his goals through ruthless means.
* Optimus Prime: While primarily the hero, Optimus Prime has been depicted as a villain in some Transformers stories, particularly when under the control of the Quintessons or other powerful forces.
Ultimately, the "bad" Transformers depend on the specific continuity and story being told.
Which "bad" Transformer are you thinking of? Maybe with more context, I can give you a specific answer!