Homunculi - sterile - eternally childless - fact
This is something that never really bothered Envy. Envy didn't want children. Didn't like children. Couldn't see the point of children when you would live forever anyway. But after some hundred years the thought begins to irk him. Some internal clock that shouldn't be there, that should have died, that ticks loud and impending and wakes him up when he ought to be sleeping.
Homunculi ≠ human -> desire for child - impossible to fulfill - unhealthy
But Envy watches mothers and fathers and how they cuddle and argue and ignore and love their children and he hates them for having what he really doesn't want anyway, but feels he should have the ability to have. Because it isn't fair that every single human, no matter how unsuitable, no matter its defects, can reproduce and he cannot.
The death of a child - Isbhal rebellion - genocide - entertainment
Envy doesn't regret it, and still revels in how it felt to kill the little snot-nosed brat. Not just for the effects that snowballed out of it, but for the act itself.
Wrath - homunculus ≠ human - companion
And Wrath is a child. Something inside Envy stops ticking when he finds the young sin. Ten, twelve maybe. Practically an infant to someone who is four hundred. And he's not human, so Envy doesn't feel the need to hate him from the start. And so he begins to shape Wrath into what he wants him to be. Takes him on and shows him the ropes, teaches him right from wrong, gives him clothes that scream of who the young homunculus really belongs to, Dante be damned.
Wrath - Envy's - child - satisfaction
Wrath takes to Envy, and the attention spent on Wrath is rewarded with adoration, affection, hugs and kisses Envy didn't realize he even wanted in the first place. And Wrath comes to him when he is uncertain and looking for advice.
"Dad," the young homunculus begins one day.
Dad - father - Hohenheim ≠ Envy
"Don't call me that," Envy snaps, and Wrath's pretty big eyes grow wide in surprise. Envy has never raised his voice to the young sin in the past.
"But…"
"If you call me that again I'll knock out your teeth."
Wrath seems to realize he means it. Envy, in turn, realizes that he doesn't like the child so much at the moment, has had enough really, wants some space. It's not as if he actually fathered Wrath anyway. He wouldn't be like Hohenheim, creating and abandoning, because he didn't create in the first place.
"Then what should I call you?"
Envy hesitates. Not mother, that's for sure. And the only other term Envy knows in connection to parenthood is…
"Master," he says firmly and glares at Wrath, who seems to think he's joking.
Over time Envy beats the message home and Wrath complies again.
Envy - master - Dante - mother ≠ Hohenheim.
Envy likes him a little bit better once more, but he can feel the strange gap that has opened up between them and doesn't know what to do about it. The fact that Wrath can't seem to act like Envy wants annoys him, and the sudden distance frustrates him, and Envy's beatings drives Wrath into Sloth's arms.
Sloth - mother - Dante - master ≠ Envy
Envy resents Sloth for it, but tells himself that it wasn't like he wanted to keep the kid around anyway. He doesn't want some obnoxious little brat. And besides, there's business to attend to.
Sloth - dead - Wrath's upset - goes to Envy -> Envy - second best
Envy is still angry, and Wrath doesn't seem to comprehend that by calling Sloth mother he is insulting Envy, which angers the older sin further and he kicks and beats Wrath, and Dante just smiles.