Before Jack shot his boss to death, he showed him his scar - his boss brutally beat him up in a bar and totally embarassed him. Jack wanted revenge, so he killed him.
The pair of antique dueling pistols dubbed ''Spanish Judges'' were hidden in Max's apartment all the time, and Max didn't even know about it.
Jack came to Jamie and Max's apartment to get their help to steal a million dollars, to kill them and steal the Spanish Judges.
Jack told the Mars Girl that he will give her money so she can leave Los Angeles so she won't be a witness.
She refused to leave, stating that she's waiting for the man from Mars.
Jack, realising that she isn't coming, twisted her neck.