Sounds like the previous owner may have had issues, ran a RUU & got rid of it. Running a RUU would put the bootloader back to being locked, even with it being S-off. It's hard to say what the issue could be, honestly. The red development disclaimer you're referring to when the phone boots up is normal on a rooted device. I'm almost wondering if the battery could be at fault, particularly since you say you can't get the phone to boot at all now. Will the phone charge at all?
Sent from my EVO LTE