i'm not into the clone wars but i think he started as a general against the clones, was almost killed, and was turned into a cyborg like vader.
*CANON*
"Grievous, born as Qymaen jai Sheelal, was the cyborg Supreme Commander of the Droid Army of the Confederacy of Independent Systems for most of the Clone Wars. Grievous was originally a Kaleesh from the planet Kalee, where he lived his early life. During the Kaleesh conflict against the Huk, Qymaen jai Sheelal quickly learned the art of war, specializing in a slugthrower rifle. Quickly amassing a great number of Huk kills, he became a demigod among his people. He eventually met the female Kaleesh Ronderu lij Kummar, a master with the sword. The two became very close before her death at the hands of the Huk. Heartbroken, Qymaen jai Sheelal renamed himself Grievous and turned all his anger and grief toward the Huk. Aided by his elite, he forced the Huk off Kalee and then swarmed their homeworld, conquering the Huk. The Huk turned to the Galactic Republic for help, and the Kaleesh were forced back to their own world by the Jedi and left to starve.
Desperate to help his people, Grievous took a job as an enforcer with the InterGalactic Banking Clan. However, after learning that the Huk had attacked Kalee, he headed home to gain vengeance. After consulting with his ally, Separatist Head of State Count Dooku, IGBC Chairman San Hill arranged for a bomb to be placed on Grievous' shuttle, Martyr. The Kaleesh was critically injured, but lived, and Hill arranged for the final stage of his plan. Grievous was reconstructed as a cyborg and was presented to Dooku as a potential weapon to be used in the upcoming war that the count had planned against the Republic.
After being made Supreme Commander, Grievous proceeded to wreak havoc on the Republic for the three-year–span of the Clone Wars. The general was trained in the art of lightsaber combat by Dooku himself, who was also a Sith Lord. Grievous was a quick study and eventually came to be recognized as one of the most skilled duelists in the galaxy. Aided by his IG-100 MagnaGuards, Grievous killed many Jedi, and led the Confederacy to victory in many battles. Quickly gaining a reputation as a savage, merciless brute as well as a tactical genius, the cyborg struck fear into the hearts of many with his flagship, the ion cannon–armed Malevolence, as well as Operation Durge's Lance, a massive strike at the Core Worlds."
*CANON END*
So let it be known, Grievous was not a coward... he was
advised to be later on... Also side note, Grievous was on Geonosis; but his task as usual was leading the Separatist leaders away to safer grounds for immediate departure.