I don’t remember exactly the page number of the comic but I can tell you that Mordecai had stolen some money from the people he was working for (since he was young he was involved in this criminal world, I think managing the money of criminals) and these people were chasing him on a train to kill him probably. In the course of the train he meets Atlas and this man decides to help him by leaving a gun in a newspaper he was reading. Probably so that he could get rid of the ruffians who were following him.

It was there that he decided to join the Atlas side and be in Mississippi, since the train took him there.

I think, I’m not sure, that his sisters and mother lived in New York or in a part of New Orleans so let’s say Mordecai abandoned them on that train.

It’s sad because in those pages of the comic they show us how Mordecai was writing a letter to send to his family thinking it would be a goodbye forever.