Marcel thought there was a future between them. No, he made the mistake of thinking that there would be one. He went to bed with various plans on how to keep Arianna by his side.
He knew it was selfish but then, he had never been a good person from the very beginning. Taking care of the Red Giant was not an issue since they were just a minor gang, but the problem was Elijah. Even as a one-man team, he's caused enough trouble for him.
Getting his guns was a priority before it escalated. He might have the chief of police on a leash but the mayor of the city would do everything to rid the streets of guns and drugs to gain good public opinion for his next tenure.
But Marcel pushed those thoughts to the back of his head as he fell asleep. He will think of a solution tomorrow. So he slept on the couch that was in his office even though his comfortable bed was not far away.