ares was physically ( and mentally, to a lesser degree ) abusive.
from the moment when he knew his twin sons could pick up practice swords, he had them training from the moment they woke up to almost sundown. ( deimos thinks that it’s because he and phobos are ares’ oldest children, thus their right to experience that. ares only conformed it. )
the training was something that deimos didn’t mind, but the punishment that came with losing to eris or enyo ( then teenagers, but still more trained than two six year olds ) left deimos licking a split lip. at least in the beginning, split lips were all he got from a backhand across the face.
once he was a teenager, the punishments got progressively worse. a broken arm, cuts to the face, a broken wrist, you name it: he most likely experienced it. even if he beat enyo and had her at sword point, if ares didn’t like his form, he’d get beat for it.
( it got bad enough one training session that artemis actually intervened. she took both deimos and phobos away to recuperate while apollo smiled & kept ares from going after his twin. )
aphrodite is/was the neglectful parent.
aphrodite knew was ares was doing from the get-go.
and, at first, the training wasn’t so bad because she was there to watch and tell ares to lay off. but then she got bored/distracted with her duties as love goddess and so the training sessions worsened because ares didn’t have to mind how hard he corrected his twins’ postures.
even when deimos came to her to complain, she was always busy with a lover and so she wrote off his complaints with absentminded agreements and hums, and empty promises to talk to ares about lessening up what he was doing.
once a mama’s boy, deimos’ affection towards aphrodite soured the first time ares broke his wrist. ( and he’s sure she felt it, the souring of love. she showed up after that, but he turned away and retreated with artemis to her own residence. )
it was something a long time coming ( especially now with prometheus having permanent possession of poseidon’s trident, a changing tide in its own right ).
sure, it took both deimos and phobos to kill their father, but after the dust settled: ares was dead, gold blood soaking into the ground. enyo with her blank face and eris with her grin ( likely pleased that he finally got what he deserved ).
and sure, his twin sons were badly hurt in the aftermath, and the gods were divided in what to do.
half, mostly ares’ children that had better treatment than his oldest sons, wanted deimos & phobos to pay. aphrodite, too, wanted her sons face up to it.
the other half, gods who openly or secretly loathed ares ( and/or aphrodite ) stepped up to get in between ares’ other children and aphrodite.
it looked like it would come to another god war, except when a silver arrow buried itself in aphrodite’s throat and she fell. ( she’d recover, sure, but with wounded pride. she should have seen artemis notch an arrow and bury it in aphrodite’s throat. ) but it was enough of a shock to everyone that all of ares’ supporters dissipated.
jury’s still out in olympus, but deimos knows that he & phobos will most likely become the twin gods of war ( + terror & fear, respectively ) now that they’ve usurped ares.