Based on another explanation I came across in the link : The Drop : Explained & Reviewed, look like they 2 were loan sharks. Based on what Bob explains in end.. he killed Ritchie for the sake of an old debt. This debt made them work for the mob as bartenders.
Before that they were independent and had their own say in things. Over time they are forced to work under the mob. Also, that is why Marv stole from the bar. He is taking mobs money. So they kill him.