Thievery is a growth industry, and thus an enterprising Thieves’ Guild administrator such as yourself will frequently be called upon to quash a rival organization. Such is the cost of doing business.
But if your opponent is half as wily as you, no amount of harassment, bluster, or clubs to the head will clear them from your path. No, if you want to remove them from the landscape permanently, you’ll have to curry favor with the various less-than-honest authorities within the city.
Victory in Antihero requires bribery, blackmail, and breaking skulls. Grease enough palms, gather enough incriminating information, and quietly eliminate enough VIPs, and you’ll soon find your rival is seeking their fortune elsewhere. Perhaps in a neighboring city. Perhaps at the bottom of the river. Either way, you’ll then have plenty of time to worry about the things that are important. Like making yourself rich. Or dealing with the next brash up-and-comer who doesn’t seem able to learn from others’ mistakes.