I think the MC's inner demon is his past life, so those markings is probably that inner demon's sins. The landowner said it's their last chance, so he probably reincarnated multiple times. I'm curious about his connection to the red head girl though. Is the reason he hated and wanted to kill specters so much because she died in their hands? If so, it's ironic that he almost killed her (even though he showed he's not interested in killing her if she did not insist).