I just brutally murdered all of the Thieves Guild leaders in cold blood for the final Fighter's Guild quest, and it has given me the Death Penalty. How can I fix this? Wiki says I mut talk to thieves guild members, but uh...lol

You can either reload your game (which I would do if possible and just taunt the leaders instead, but I'm guessing you already saved?), or use the console command "SetPCCrimeLevel 0" to get rid of your bounty. Try to be more careful next time

Reload and frenzy em, or just kill every guard in sight as they come to kill you

Try to be more careful? Ｔｈｉｓ was a final quest, on behalf of a guild I had every reason not to distrust. Moreover, I was told that an arrangement with the Comonna Tong was made, so I figured I would be absolved or something. I am blameless I say. Blameless!

No, you are to blame. You can't go murder anyone in the way you like, especially with non-hostile NPC's. As you already said you brutally murdered them. Well, there you have it. If you make the first blow, then you violate the law. And thus it result in a nice bounty. Killing more and more people this way and you have the death penalty on you. I wonder how you did the quest from Larrius Varro then. So yes, be more careful. Instead of going berserk all the way taunt them and let them do the first blow. In that way you avoid any bounty. With hostile people it doesn't matter and you can go berserk.
By the way; i would go for the route of percius mercius, instead of that camonna tong puppet.

I wonder if you've done it at all. It would be your only way out of death penalty now that the Thieves Guild members are dead. If you haven't done Larrius Varro Tells a Little Story, do it, and he will get rid of whatever bounty you had even prior to it.

Otherwise you are stuck if you're on console or if you want to play it legitimately on PC.

The Cammona Tong and the Thieves Guild are enemies. Several of the higher ups in the Fighters Guild are in the Cammona Tong's pocket. This isnt about reducing the crime in Vvardenfell, since the Cammona Tong is a criminal syndicate, its about eliminating the competition. I'm not that far in the Fighter's Guild questline, so I cant check the dialogue or journal entry as to the details of the "arrangement". The wiki does say that he gives you gold to pay off the bounty, though. You'd probably have to kill the leaders one at a time, then run outside before any other guild members can get to you, then pay off the fine. Of course, your best bet is taunting or frenzy. A custom 2 second, magnitude 70 spell should work, and still be somewhat easy to cast.

There are only 2 which are really involved in the camonna tong and that's the current guildmaster and his orc sidekick, which both reside in vivec. Eydis isn't a fully camonna tong supporter, as she will betray them if you bring her the bittercup (she turns out to be a clavicus vile worshipper)
Percius Mercius is the old guildmaster and the opposite of sjorring. Hrundi sides with Percius, so that's a good guy too.