All right, so I spent my day charging through FG quests to test out what's going on with our little fetcher.

Here's what I've got. On killing Maglir, I
was expelled from the guild, although I got no quest pop-up saying as much. After Maglir was killed the Fighters' Guild was no longer listed on my faction screen, and speaking to any member of the guild after that results in the "you killed a member; go speak to Vilena Donton" speech.
Using FormID on Maglir during "More Unfinished Business" returns the following, faction-wise:
Faction: Blackwood Company - 0002FF29 - Faction is EvilFaction: Skingrad Citizen - 00028E77 - Faction is HiddenFaction: Leyawiin Faction - 000338EC - Faction is HiddenFaction: Chorrol Citizen - 00028D98 - Faction is HiddenFaction: Bravil Faction - 0002CA32 - Faction is Hidden
SameFactionAsPC returns 0.00. None of my other mods seem to be modifying him, as FormID confirms he's from Oblivion.esm, as are all the factions he's a part of.
... and now, during "The Hist," getting this info as he's charging at my character's face:
Faction: Fighters Guild - 0002228FMod: 00: Oblivion.esmFaction: Blackwood Company - 0002FF29 - Faction is EvilMod: 00: Oblivion.esm
SameFactionAsPC also returns 1.00. So, something's added him back into the Fighters' Guild faction when he obviously shouldn't be.
And FWIW, my modlist:
Spoiler Masters for: Save 26 - Amarel Mathmeldi - Modryn Oreyn's House, Level 27, Playing Time 171.32.06.ess00 Oblivion.esm01 EnhancedWeather.esm [Version 1.4.1]02 Cobl Main.esm [Version 1.72]03 Kvatch Rebuilt.esm04 Unofficial Oblivion Patch.esp [Version 3.3]05 Oblivion Citadel Door Fix.esp06 DLCShiveringIsles.esp07 Unofficial Shivering Isles Patch.esp [Version 1.4.0]08 EnhancedWeather.esp [Version 1.3.5]09 Enhanced Weather - Across-The-Borders Mods Patch.esp [Version Final]0A All Natural - Real Lights.esp [Version 0.9.9.5]++ Advanced_Water_Modification-2734.esp0B Atmospheric Oblivion.esp0C Distant Chapel Bells.esp0D WindowLightingSystem.esp0E SoT_Holiday.esp0F Enhanced Economy.esp [Version 4.2]10 Crowded Roads Revisited.esp [Version 1.1]11 FF_Real_Thirst.esp12 Map Marker Overhaul.esp [Version 3.5.1]13 Map Marker Overhaul - SI additions.esp [Version 3.5]14 DLCHorseArmor.esp15 DLCHorseArmor - Unofficial Patch.esp [Version 1.0.5]16 DLCOrrery.esp17 DLCOrrery - Unofficial Patch.esp [Version 1.0.3]18 DLCVileLair.esp19 DLCVileLair - Unofficial Patch.esp [Version 1.0.5]1A DLCMehrunesRazor.esp1B DLCMehrunesRazor - Unofficial Patch.esp [Version 1.0.4]1C DLCSpellTomes.esp1D DLCSpellTomes - Unofficial Patch.esp [Version 1.0.1]1E MaleBodyReplacerV4.esp1F TFF_Races_Base_RSM.esp20 Slof's Male Clothing.esp21 UFF Armor of Gaia.esp22 TFF_Armor_Base.esp23 TFF_Clothing_Base.esp24 TFF_Clothing_SI.esp25 Accompt of the Delitescent.esp26 UFF & RM Assassin Elite Armor.esp27 Pirate Gear.esp28 UFF - Celtic Witch Robes.esp [Version 2.1]29 UFF RM Ranger Armor.esp2A Slof's Horses Base.esp2B Slof's Extra Horses.esp2C DLCThievesDen.esp2D DLCThievesDen - Unofficial Patch.esp [Version 1.0.5]2E DLCThievesDen - Unofficial Patch - SSSB.esp [Version 1.0.4]2F Cobl Glue.esp [Version 1.72]30 Cobl Si.esp [Version 1.63]31 FF_Real_Thirst, Cobl.esp++ Cobl Tweaks.esp [Version 1.44]32 Ivellon.esp [Version 1.8]33 Better Benirus Manor for Cobl - Complete.esp34 BHC_Expanded.esp [Version 1.2]35 hackdirtabyss.esp36 ImpeREAL City Unique Districts - All the Districts - Merged.esp37 Kragenir's Death Quest.esp38 KDQ - Rural Line Additions.esp39 Kvatch Rebuilt.esp3A Kvatch Rebuilt Weather Patch.esp3B Slof's Oblivion Goth Shop.esp3C The Ayleid Steps.esp [Version 3.4]3D UFF Gypsy Market.esp3E Cyrodiil Travel Services.esp [Version 2.0.3]3F DLCBattlehornCastle.esp40 DLCBattlehornCastle - Unofficial Patch.esp [Version 1.0.4]41 DLCFrostcrag.esp42 DLCFrostcrag - Unofficial Patch.esp [Version 1.0.4]43 Knights.esp44 Knights - Unofficial Patch.esp [Version 1.0.9]45 SM Plugin Refurbish(Merged).esp [Version 1.30]46 TOTF.esp++ TOTF Delayer.esp47 RTT.esp++ Enhanced Weather - Ruined Tail's Tale Patch.esp [Version Final]48 Blood&Mud.esp49 Blood&Mud - EE patch.esp [Version 4.2]4A Shadowcrest_Vineyard_COBL.esp4B Shadowcrest_Vineyard_GuardCOBL.esp4C xuldarkforest.esp [Version 1.0.5]4D xulStendarrValley.esp [Version 1.2.2]4E xulTheHeath.esp4F XulEntiusGorge.esp50 xulFallenleafEverglade.esp [Version 1.3.1]51 xulColovianHighlands_EV.esp [Version 1.2.1]52 xulChorrolHinterland.esp [Version 1.2.2]53 xulBeachesOfCyrodiilLostCoast.esp [Version 1.6.4]54 KragenirsDeathQuest-LostCoast patch.esp55 xulBravilBarrowfields.esp [Version 1.3.2]56 xulLushWoodlands.esp [Version 1.3]57 xulAncientYews.esp [Version 1.4.3]58 xulAncientRedwoods.esp [Version 1.6]59 xulCloudtopMountains.esp [Version 1.0.3]5A KragenirsDeathQuest-CloudtopMountains patch.esp5B xulArriusCreek.esp [Version 1.1.3]5C xulPatch_AY_AC.esp [Version 1.1]5D xulRollingHills_EV_withoutWheat.esp [Version 1.3.2]5E KragenirsDeathQuest-RollingHills patch.esp5F xulPantherRiver.esp60 Blood&Mud-PantherRiver patch.esp61 xulRiverEthe.esp [Version 1.0.2]62 xulBrenaRiverRavine.esp [Version 1.0.2]63 xulImperialIsle.esp [Version 1.6.5]64 xulBlackwoodForest.esp [Version 1.1.0]65 xulCheydinhalFalls.esp [Version 1.0.1]66 Blood&Mud-CheydinhalFalls patch.esp67 TearsOfTheFiend-CheydinhalFalls patch.esp68 KvatchRebuilt-CheydinhalFalls patch.esp69 DungeonsOfIvellon-CheydinhalFalls patch.esp6A xulAspenWood.esp [Version 1.0.1]6B xulSkingradOutskirts.esp [Version 1.0.1]6C BarrowFields-Blood&Mud-Patch.esp [Version 1.3]6D AutoBookPlacer.esp6E Curseman's Anti Extinction Mod.esp6F Dungeon Actors Have Torches 1.6 DT.esp70 seorin's Artifact Fix.esp++ sixier Sheogorath.esp71 Enhanced Vegetation [100%].esp72 Enhanced Economy - House prices.esp [Version 1.0]73 RealSleepExtended.esp [Version 2.4.1]74 nGCD.esp75 nGCD Oghma Infinium.esp76 nGCD Skeleton Key.esp77 1em_Vilja.esp78 bgBalancingEVCore.esp [Version 10.52EV-D]79 bgMagicEV.esp [Version 1.7EV]++ bgMagicSpellTomes_for_WryeBash.esp [Version 1.68EV]++ bgMagicItemSigil.esp [Version 1.68EV]++ bgMagicEVAddEnVar.esp [Version 1.68EV]7A bgMagicEVPaperChase.esp [Version 1.68EV]7B bgMagicAlchemy.esp [Version 1.57]** bgBalancingEVOptionalNPCDiversity, TNR Merge.esp [Version 10.0EV-D]** bgBalancingOptionalNPCDiversityKvatchRebuilt, for Wrye Bash.esp [Version 9.0EV-D]7C bgBalancingEVLAMEAddition.esp [Version 10.51EV-D]++ bgBalancingEVLAMEGlue.esp [Version 10.0EV-D1.62EV]** bgBalancingEVOptionalNPCDiversityLAME.esp [Version 10.5EV-D]++ bgBalancingEVOptionalSeamReducerHighElfFix.esp [Version 10.0EV-D]7D bgIntegrationEV.esp [Version 0.993]7E TheOubliette.esp7F bgMagicEVShader.esp [Version 1.7EV]++ bgMagicShaderLifeDetect.esp [Version 1.68]80 bgMagicLightningbolt.esp++ KoldornsImprovedLava2.esp81 Enhanced Economy - Soulgems.esp [Version 4.2]82 Real Hunger, Cobl.esp [Version 1.6.1]83 StokerWolff.esp++ Cobl Filter Late MERGE ONLY.esp [Version 1.53]84 Bashed Patch, 0.esp85 Oblivion Graphics Extender Support.esp [Version 0.2]86 BetterCoins.esp87 DLCFrostcrag_MysticEmporiumPatch.esp88 FormID Finder4.esp
I'm hanging onto a save from just before my character accepted "The Hist," so if there's any more testing that needs to be done, let me know. If I get really ambitious I may start waving Wingrep at things and see if I can find any other mods that might have their sticky paws all over Maglir. Probably won't happen tonight, though. My eyes are starting to cross.