Yeah, it'd be kind've cheesy if the dark apprentice just popped out of thin air and stabbed Starkiller..
I believe this Galen Marek is a clone. Just because he had memories that the original Starkiller had doesn't mean he IS Starkiller. Remember, the clones of Jango Fett would often have memory flashbacks from HIM; and he died in Episode II.
"If at first you don't succeed, hire a mercenary to do the job for you. At least they'll get it done right."