Although Michael isn't too self-conscious when it comes to his scars, he is glad that most of them can be hidden under a shirt. Most of those scars - the one on his lower back, the one on his right shoulder blade, the one on his abdomen, and even one that's on his left side - belong to another life, and it's easier to avoid too many questions if they can be hidden. He's not ashamed of them, he's actually very proud of them because they represent battles that he has faced and won, but they're just not something that he likes to go around showing off to everyone. They're something that he considers as personal, something that's just for him to carry.
No matter how much he'd like to hide them all, though, there are some scars that he can no longer hide under clothes; Jack and Charlie had made sure of that.
Three months ago, after his old bosses had held him captive so they could 'teach him a lesson in loyalty,' he had been left with some wounds that had left his body scarred. Along his jaw, on his right side, there was the line of a scar from a cut he had received in one of those nights that he had been held captive. It's not as noticeable as some of his other scars, and most of the time he figures that people can't even really see it unless they were to stare at him, so he cares about that scar as much as he cares about the faint scars along his right knuckles: They're just there, they're not that important.
The one that's the most important to him (and the most obvious) is one that is on his right inner forearm, running across the
tattoo of a scorpion that had once represented more than he ever let on. His memory's still hazy on how exactly that wound had been done, but to him that was the least important detail.
That night, when he had gotten that tattoo, had been his first night of officially working for Jack. Maybe it wasn't that big of a deal, but for Michael and his friends it had represented the beginning of something big; maybe one day one of them would be able to take over the group, maybe one day they would be the leaders rather than just the followers... It had been a night when Michael and the rest of his friends that had also just started felt invincible. After going out for drinks they had gone to a tattoo parlor where they had each gotten a tattoo, to signify the beginning of a 'new generation.' The beginning of something great.
Of course, as most things do, everything ended and after a few years the group had disappeared and they had each gone their own way. Michael had changed directions the most out of everyone, though, and instead of staying close to Jack how almost everyone else had done, he had instead turned his back on him and had led the authorities right to him. To Jack that had been the worst type of betrayal possible, especially because he somehow figured that Michael wouldn't even be alive if it wasn't for him and his help, so it was because of this that he had done his best to make Michael's life a living Hell. The scar running across the tattoo had been done so that Michael would never forget that he had once belonged to that group, and that he had betrayed them. That, because of that betrayal, Jack believed he had the right to continue making his life Hell until one of the two was out of the picture.
Now that he's been free for three months and that he's had time to think about everything that happened, Michael knows that it's no coincidence that he wasn't killed throughout the nine days he spent being a prisoner in Jack's little 'game.' He knows that it wasn't luck what helped him and prevented him from bleeding out from the wounds he had received - he had been kept alive, because Jack wanted to continue this game until one of the two got tired of it.
That wound on his arm that had left behind a visible scar for the whole world to see was just a a sort of bullseye, reminding Michael that it was just a matter of time; that his past actions would eventually get him killed. As Michael saw it, though, the scar just reminded him that he and Jack had a few things they still need to settle, and they remind him that he can't rest until Jack is officially gone from his life, in every sense of the word.