He was scaring her, Marcel could see it in her eyes, but he did that on purpose. Arianna had to know that being with him wasn't going to be all sunshine and rainbows. Moreover, he was not her typical male with a high moral principle, but he had his moral compass in check.
In one word, he would intentionally hurt her sometimes with his decision, that was a given considering his occupation, but he would try his utmost best to make Arianna happy, and the one thing he would never do is to lay a hand on her.
As the Mafia Lord of the Luciano clan, he would have to keep up with his reputation, but Marcel knew the clear difference between respect and fear. Arianna would respect him, but there would be no need to fear him. He would have to earn that respect as well.