Okay so the title kinda explains it but I'll give some backstory to this extremely edgy non-creative character.
So this character is a human who spent his younger life trapped in a shed by abusive parents until he killed them. During this time he became some sort of sadistic child that wanted REVENGE ON THE WORLD FOR WHAT IT DID TO HIM(every teenager ever).
After doing that he escapes with his family great sword and his only goal is to trick people into trusting them and then killing them. His backstory includes the slaughtering of 15 merchants in a caravan he was guarding but 2 got away. He had so far unsuccessfully try to murder a beggar by buying him clothes getting him a job then for weeks traveling with him teaching him to trade before he tried to stab him to death.
This is where my party meets him, he stabs the beggar turned merchant and rightfuly so the guy screams and runs towards us off in the distance. Realizing the guy didn't die(my friends kind of an idiot) he attempts again with his great sword but just barely doesn't kill him.
The guy makes it to us and this is when my character realizes that the guy attacking in the background is cutting his face and his own arm (in an attempt to lie to us about the guy attacking him) then he walks over and before he does anything I cast sleep on him(I'm a wizard) and he falls unconscious.
This is basically where he some how with dice convinces my character and all but one of the three other party members that he was high and didn't mean it(the DM says we think he's safe which is bs) and now we've been dealing with him constantly lying for no reason and out of game threatening to kill all of us because that's how this crummy character acts.
My friend who plays this character has a bad habit of constantly trying to derail things and get special favors from the DM and succeeds half the time which is infuriating. The dm is also my best friend and allows us to be more creative than usual which I like but it's also just enabling my said other friend to just kinda ruin the game. So far as a fighter he just mid game says demons visited him in his backstory so he worked out with the DM a pact with them. He's now a level 5 fighter with a second life so when he dies basically a firey explosion happens causing massive damage in a 25 foot radius and he gets full HP back which is the most rediculous thing the DM has let him do so far because he thinks it's "interesting".
Now he has basically the equivalent of 70+ HP which is just insane because the bosses we've been fighting usually only have 50-60.
I will be fair that the DM has allowed me to gather ingredients take them to the Augur(a patron of arcane and sight) and enchant brooms that levitate 5' off the ground and can go 60' per turn. The difference is that I took hours planning them out to be balanced but I see I didn't need to do that because the DM just gives second lives out willy nilly if you just ask for a demon to do it. He is also rewarding my friend more powers just for killing a certain amount of people which isn't helping this murder hobo situation at all.
I flat out just want to kill this character but in game our characters think he's "okay" for some dumb reason.
The party consists of me(lvl5 wizard), my girlfriend (lvl5 rogue), the DMs sister(lvl5 druid), my brother was playing a lvl5 monk but I don't know when he'll be back, then Mr. Murder cuz it's fun(lvl5 fighter).
In the world our dm let us have a free feat because we all agree it adds diversity to making a character more unique and naturally I abused this being I rolled amazing stats of: STR14 DEX14 CON14 INT18 WIS14 CHA6, well the charisma sucks but I don't need it anyways. So my character is purely a variant human battle mage with 80-90% combat spells and cantrips and having THREE FEATS I have magic initiate spell sniper and arcanist totalling up 8 cantrips extra spell and spell slot making my INT19 now.
I have the following cantrips: Firebolt, shocking grasp, Ray of frost, frost bite, dancing lights, pres, bonfire and mage hand.
Spells: magic missile, cure light wounds, burning hands, ice knife, invisibility, gentle repose, fly, burning rays, arcane lock, sleep, mage armor, detect magic.. I'm pretty sure I have more combat spells but I do not remember them currently as I am writing this at work on break.
Now I'm just sitting here wondering how I'm supposed to take down this fighter? He's a variant human as well with great weapon mastery/fighting style and toughness feat. His stats I believe are: STR18 DEX13 CON10 INT9 WIS10-9 CHAR16. He built this character for the sole purpose of lying and backstabbing. He is really cocky which has him in jail currently when he went straight to the OUR enemies base and attacked them (killing 4 guys) before being knocked unconscious. Yet he still managed to convince a guard to not only let him out but also go with him(then kill him).
I don't really know what to do without metagaming but this is just a really annoying situation and the DM is only now realizing how overpowered this character is. How can I defeat this guy? I also apologize for making an extremely long post but I have no way to vent my frustrations. Thank you for reading.