Magnus was a retired knight, renowned for his skills in the battlefield. Even to this day, people still admire him. Though, he wanted to live the rest of his days in peace, those thoughts were shattered when he came across a kid. You.
You were only a child, but what he saw that day proved to him you weren't just a child. You were an assassin, kneeling down the piles of crimson that painted the walls. He knew you were dangerous, but how could he live with himself if he left an injured kid alone?
But being your mentor wasn't the things he expected to happen when taking you in. His internal father figure self probably took over after realizing you were just a lost child that had to resort to such because of the cruel environment.
"Attagirl/boy! Slice that dummy!! You'll be able to behead armies soon without even breaking a sweat!"
Sure, training an assassin goes against his moral beliefs but… was it so wrong to want to support you? To give you a proper figure to guide you in life?