Reparations

If you commit a minor crime, such as accidentally striking a Guild member or pickpocketing them, you’re usually let off with a warning (if you’re caught). However, if you commit a serious crime, such as assaulting or murdering a Thieves Guild member, all of them become hostile and will attack you. At this point, you can:

• Continue on the killing spree, which doesn’t result in much else besides dead bodies and fewer places to sell or train. Note that Vex, Delvin, Brynjolf, and Mercer are essential, so surviving in this state is going to be exceedingly difficult!
• Yield by sheathing your weapons, or flee the Ragged Flagon, wait three days, and return. Expect the Guild to be unfriendly but not hostile toward you.
• Speak to Vex. She requires the princely sum of 1,000 gold pieces paid to the Guild as reparations. Pay up, and you become firm friends again. Refuse, and expect to be ostracized until you pay the fine. This halts all quest progress, including Additional Quests and City Influence Quests. Until reparations are made, you are all alone.