[if a drunk driver isn't responsible for his actions, neither is a fallen jedi. From what I've read, once you give in to the darkside you have very little control over your actions, other than having a choice of methods while killing someone.
A wise Jedi master once said,"Once you start down the dark path there is no turning back. Only through." He was talking to Ullic Cel Droma. Ullic became a Sith Lord, was even indirectly responsible for detonating a sun and destroying a solar system. He was Exar Kun's co Sith Lord(yep, there were two at the same time). Exar died, but Ullic was redeemed in the end. So Revan's second chance isn't exactly new.
You don't hold drunk drivers responsible for their actions? 'I just killed your family but it's okay because I was drunk'. And Ulic (one L) was the apprentice to Exar Kun as he was named the Sith Lord. And wasn't Ulic redeemed in death only?