You're right. No need to overthink.
I think you said I was working at a low level for the mobsters. If the mobsters I was working for are the ones that committed the murder, and assuming I knew they were mobsters, I would have to cover it up. After all, I was collecting a paycheck from them despite knowing what business they are in. If Tony offered me $10 million to do it, so much the better. I I worked for the mobster that got murdered I would tell the others in the organization and let them kill Tony and other mobsters in that organization. In that way I would be in the good graces of the mobsters I work for, and the mobsters in the other group would be dead and I wouldn't have to testify.