Leaf fans have wanted Marner gone since the Montreal series
1) No heart
2) Scared to get in the dirty areas
3) Cracks under pressure
4) Scared to take a hit, turns pucks over as a result or flips it over the glass
5) Extremely entitled
6) Massive ego, can't take critcism
7) Refuses to take any accountabilty
8) Extremely poor "leadership" skills
9) Doesn't perform in big games
10) Is being paid like a top player in the league
He is simply not a player you can win with. I've said it about a lot of other players in the past, he is one of them. If he is one of your "leaders" you are doomed. If he's on a team where he isn't wearing a letter, there are other veteran star players that take the lead and he can sit back and be the 3rd/4th option guy, he can be successful. He's just not what he's being paid to be, nor what he thinks he is.
Prime example, Eichel in Vegas. He had many of the same issues and poor habits/attitude as Marner in Buffalo. Gets traded to Vegas, where they have an extremely strong leadership group, guys who have won multiple cups, elite players that compete at both ends, where there isn't all the pressure in the world on him to perform, and he blossoms.
I think it's best for both sides to part ways.