He loved her. Not in the ordinary sense, to be sure. It's quite a different connection than the average couple can attest to. He felt her. Her pain, her joy. Emotional or physical, he felt it all. From the smallest prick of the finger, to the darkness of overwhelming grief. The same went for an embrace, or her merriment from winning a Quidditch match. But whatever she felt, he received it twice as strongly. Her being in pain nearly killed him and whatever joy she sensed filled him with such contentment. Her face was in his dreams every night. He grew restless when she was too far from him. He needed her just as he needed oxygen and he needed her safe and happy.
His father called it a curse. "When the Flint men fall in love, it is equivalent to burning in hell." Not that he knew about her, he would be furious. Angelina Johnson wasn't exactly the type of companion his father had in mind for his only son; an ally to Dumbledore, dedicated to the purging of the Dark Lord. And yet, Marcus knew he could never love anyone the way that he loved Angelina. He also knew that there was absolutely no way that he could be with someone else so long as she drew breath. He would lie to his father to keep her safe.
He loved her so deeply, but she barely noticed him. In some ways it was easier for him like that. So long as she wanted nothing to do with him, there was no need for her to be dragged into his hectic and dangerous life. He also didn't have to explain his feelings for her to anyone. But there were also many drawbacks to her indifference. Like the times when he wanted nothing more than to hold her until her pain deceased, or the times when he needed to hear her voice or see her face in order to breathe again. How she tortured him and yet soothed his very soul. Ironically, the only person who could possibly understand his situation was the one person he would never tell.
His father too once loved as he did. William Flint loved his wife Averill so much that it killed her, that he killed her. Averill disapproved of William's involvement with the Dark Lord. It hurt her deeply to see her husband destroy so many lives. Averill's pain tore at him for many years until one day, he couldn't bear it anymore. Now, William's connection to the Dark Lord was much too strong for either Averill or their son to walk away unharmed so, he did the only thing that he could think of to prolong his Averill's old life. He killed her and she was buried behind their manor so that William could visit her often. He was relieved that she no longer felt any pain, but his heart was never whole again. Marcus was only five at the time. It sickened him to think that he may be capable of committing the same heinous crime against Angelina. After all, it was in his blood. No. It won't happen. He would be different, he would be stronger... he had to be.