Mason grew up as a bomb just waiting to destroy everything in his path. His mother didn't try to conceal who his father was, what his heritage was. She was always upfront and honest but was never concerned for what dark trail he might walk. Hades too had no problem showing himself off and taking him to the Underworld when he felt like it, feigning interest in how his day has been, what happened at school, all to see if Mason could be the prodigy he wanted and embrace his powers. So an innocent boy turned into a teen with an ego, powers of the God coursing through his veins; a little dark and a little mysterious and handsome. He lapped up the attention though he could be cold and dismissive to people that didn't interest him. That cold nature amplified due to many losses of anyone he grew a warm heart for. Eventually, he decided three separate occasions were enough to quit trying to appease others or play into humanity.
The first instance was when he touched his girlfriend, leaning in for a tender kiss, and what was meant to be an affectionate act when his lips met hers, turned out the coldest act, as his first kill. From there started a spiral of emotions and the haunting memory that he couldn't recover from. His mother tried to shake him out of his depression, playing the best comfort and support she could and get him back on his feet. Just as he started to take her words in and let the tragedy fade to the background, and trust himself again, he wrapped his mother up in a hug in appreciation for her efforts and tolerance only for her to die in his arms.
The third was deliberate. Cold and deliberate with little regret. He still had his sister, Rebecca, but after her boyfriend impregnated her with a child, he decided to cheat. After all the innocents Mason had taken, and girls he were fond of, he decided to do his own justice. Why should they die and he live? So with a grip on his upper arm and mock friendly smile, Rebecca's boyfriend drew his last breaths.

Hades expressed pride and comfort in one and it tightened their bond. So much so that Mason let Hades persuade him to attending a special camp with his older sister, Jennova. He had always stuck to the shadows, practiced his "dark powers" while Zeus's child absorbed the light, but Jennova was his sanity. His only means of company, acceptance and slither of humanity and blood and assurance. She was a reminder he wasn't damned and he was never alone. That there was someone out there that fully understood what he was going through. Then came another one of Zeus's self-righteous children and she decided to execute Jennova. Behead her in fact while Mason was tracking an older-demigod for the war. Mason didn't need more reason to be so hateful and cold but Alex gave him that. Mason responded by trying to claim Alex's life, slowly, but too many do-gooders were around to get the job done.

Seemingly at the worst time, after the war and death of a friend, Hades called upon Mason to join him in the Underworld. Take refuge and power, learn the chores of ruling the Underworld. Mason knew it was the best place for him to be so abandoned the camp and its dramas. In the Underworld Hades spent his time rewiring Mason to forget about worrying how the people above were doing and reinforced to prioritize himself. Hades taught him to be cold at the core and put on a show for others, but never lie, simply debrief. Hades taught him the wonders and great potential of his abilities. Taught him to kill mercilessly and look the poor soul in the eyes as Mason passed them to the Underworld.