Context: Mammon was sent to jail for treason after attempting to overthrow Lucifer once he got too greedy to cope with the despair and bullying of his brothers. Lucifer and Mammon both believed that the younger brother would be sentenced to death, hence Lucifer does one final favour for Mammon and personally beheads him. Shortly after he discovers Diavolo didn't intend to kill Mammon and the brother's are incredibly distraught by this.
Trigger warning for blood, self loathing, death, grief, murder implications
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He can't— no, he wouldn't sleep.  He could sleep, but he was felt haunted.  Sins crawling up his spine, pushing into his scars, then holding his throat.  He couldn't breath, it was difficult to, more than before.  The motives are different now, from work to Mammon.  He could remember it all; the scent of iron, red dye, the cold handle, his brother's head, the last words.  No matter how hard he tried to wash his hands he could still feel the uneasy, red warmth dripping off his hands.  How could he sleep?  He's a murderer.  He's more conscious of this than ever before.  He killed his brother with his very own hands.  He loved Mammon more than anyone, yet he also caused his demise.  Ironic, isn't it?  It's been even harder to look at the red wine now.  And the hatred in his glares has became even more obvious.  What a pathetic older brother, who can't even protect his younger siblings, just like before.  What has he been doing his whole life?  He isn't protecting anyone despite his wishes.  He loathes his weakness, his pride, his scars, himself.  The one that should've died was him, he's meant to be a protector after all, but he couldn't even do the one thing he lives for.  And now, another precious member is gone.  No amount of ripped wings can save them, and no amount of work can let him go back in time to fix his mistakes.  Mammon is dead and it's all his fault.  Lucifer lets out a sigh, and no tears came out.  Did he even deserve to mourn?  To grieve?  He didn't know if he even had the right anymore.  Maybe he's doing everything wrong.  He couldn't tell.  Lucifer gets out of bed, fixing his outfit before going back to his desk.  Work, what he needs to do is work.  Maybe that'll get his mind off of what happened.  Maybe... when he works, he doesn't concentrate on anything else after all.  He.... he's still tired.  But he may as well do something productive while "relaxing"