He had always scared them. There was always something…just not quite right about him. As children, neither of them had understood that and thought that it was simply because he was evil.
Now, as Elrond looks back, he finally understands that it was so much more then that. Maedhros Fëanorian was so much more complex then they had ever imagined. There was a deep and unhealable hurt that festered deep within him, a hurt that had formed possibly ere the Sun and Moon rose for the first time.
Though Elrond, with his inside knowledge, had a feeling that though he had been broken after Thangorodrim, Fingon had been there to put him back together then and to be his rock and because of that, he had healed.
It was an enormous wound, but a well-healed one and now just a mere scar that you had to know where it was to find.
But… after Nírnaeth Arnoediad, when that rock was dead and gone, there was no one and Maedhros had been forced to heal himself. Deep in a mourning that Elrond now thought he'd actually never left, even if Maedhros himself thought he had, the healing had been sloppy and his soul was covered with cracks and scars that had never fully healed and never would have, maybe not even now in death.
Privately, Elrond berated himself over and over again for refusing to ever let his guard down around the elder Fëanorian. Maybe if he had, he could've at least soothed those never-ending hurts. But as an ignorant elfling, he had only seen the outer shell of the kinslayers and was ever nervous and defensive around him.
It had taken that elfling centuries to grow up and grow out of his protected little world where everything was grouped into good or evil, and black or white; and see that that shell had been Maedhros' last line of defense and if he had only probed it enough, it would've let him in and he would've seen the broken ma-child beneath.
Maglor had let them see his inner self without pressure, and that was why Elrond and Elros had accepted him so readily as their father figure. But Maedhros had had a sicker, sadder and more broken soul underneath his shell. Though, that would've caused him to be much more cautious and maybe he had forgotten how to trust. It had seemed to Elrond that Maedhros shoved even his brother away; didn't even trust family.
And Elrond wonders if Maglor had ever even seen the scars beneath.
He doubts it.