Mankar Camoran unmodded is in an Evil faction, so killing him should not count as murder unless another mod has changed his faction.
The three Mages Guild NPCs are all set to essential initially, but once the battle begins they cease to be essential, to encourage the player to work to keep them alive. You can be expelled from the Mages Guild for attacking a guildmember, but you say you did not accidentally harm any of them, and the reason given is killing, so I can only guess that again another mod has changed the conditions for expulsion/broken the conditions.
"Ah ha" as a random dialogue subtitle is from a mod, absolutely no dialogue in the vanilla game contains that two-word sentence.
so you have one or more mods installed which make changes they probably shouldn't do, if you want to resolve this you'll need to track those mods down either by loading all into TES4Edit and checking the records you know about, or else by disabling mods and testing to see if it still happens.
Checking in TES4Edit nothing but the usual suspects (OOO, UOP, Cobl Races TNR) is affecting the MG NPCs. Hm. In the beginning of the Camoran battle my char killed the dude that follows you there and IIRC that can cause a murder count - but the message turned up much later, just after killing Camoran. Might have been a question of "queued" stuff perhaps as it was rather busy. But that still leaves the MG "killing" issue.
Have you done the Thieves Guild questline? Or at least part of it? There's a mission where you have to break into Fathis Ules' (spelling?) tower and steal his staff or something (He's a prominent member of the MG, though not essential). There's also a good chance that you wind up killing him during that quest. Happened to me, and since I was a member of the Mages Guild at the time I had to spend ages searching for all those daedra hearts and vampire dust.
Nope, haven't touched the TG questline.
The 'ah ha' subtitle is from when NPCs are fatigued or injured.
Can't recall if it just Realist Fatigue or that and MMM extra wounding.
Thanks, it was RF Pants from Realistic Fatigue.