okay so the bar brawl turned into a massacre, I even had Kenze and some of the Xanatar guilders run away, only for the party to kill them with spells and javelins. One of them is friends with Jalaster and he said he’d try to deal with it. 

The problem is that one of the guilders survived and was arrested by Jalaster, I’m assuming that the guilder would report the murder of his friends and this would mean that the party need to be arrested. 

Now I can either charge the party with unjustified murder or justified “murder “ , please help me out as I’m not sure how to proceed 

Thanks for the advice,  in answer to your questions this all took place in Waterdeep 

1. The guilders started the bar fight with an NPC, According to "The Code Legal" section IV, Assaulting a citizen is a crime, though the watch overlook drunken bar brawls sometime

2. The adventurers "drew "first by casting fire bolt 

3. & 4. Murdering a citizen without justification is punishable by death or 10 years hard labour, plus fines, while murdering with justification is exile, hard labour or a fine

Now i don't want to kill my PC's as this all happened in session 1 and having them undergo a prison sentence while accurate doesn't make for the best story , there is one thing that they have going for them and that is the guild are a criminal gang rather than a actual guild and there's been a lot of violence between them and another gang, which means that the watch are generally happy to turn a blind eye, but i do want to scare the PC's so that they think twice about becoming murder-hobos 

 so any further thoughts and ideas would be appreciated 

cheers

Ash

Cheers dude, Yeah you're right, I'll have  them arrested and then see what happens 

Read ahead. There is a lot about corrupt guards and such. Heck in the next encounter at the Zhent hideout the guards show up and decide not to arrest them. You control the world. Make it happen how you think they will enjoy it. Maybe they do get arrested and the Bregan or zhent factions offer one of their chapter 2 quests as a way to buy themselves out of trouble. If they take the job, the agent makes it all go away. etc...

Yeah i read that i wasn't sure why the characters aren't arrested at the Zent hideout, considering in later chapters they can be arrested for something substantially less serious depending on the villain

Im tempted on running the blue alley adventure as a way that the issue can go away, afterall Mirt does have some ooomph in waterdeep

It all depends how and where you want your story to go.

Maybe they meet a gang of thugs in prison who orchestrate a break-out.
Maybe a mysterious benefactor pays the guards/fine off under the obligation that they perform a "special errand".
Maybe a shady guard/lord partakes in slave-fighting and sells or signs them up secretly for combat events.
Maybe a plague breaks out and as prisoners die, some begin coming back as undead, overwhelming the guards.
Maybe the 'courthouse' catches fire during their sentencing and they do X, Y, and Z.

You can introduce story points which can provide opportunity for almost anything.  Let the characters be punished for poor decisions, but don't let the story bottleneck because of such.  Give them penalty or a price to pay to add weight to the situation.  (I had a player once lose a finger after having gotten caught stealing.)

The story is what you make of it.  : )

Honestly, having them get arrested and do ten years hard labor only to pick the story up when they get released ten years later sounds awesome.