I have an impression that as a talented player he has quite an ego and therefore when coaches don't use him properly (from his point of view) he gets unmotivated. So what happened in the KHL is basically that after quite some struggling Hartley found a role that was fine both for Kostin and coaches - an extremely aggressive hard-hitting middle-six powerforward with a decent amount of offensive responsibilities and Kostin really succeeded there. I guess he wasn't that impressive back with the Blues, but just a simple big body role likely isn't too motivating for him.
You have more experience with player than I do but as outsiders none of us know the players well enough, or at all to conclude things like "head case". Players get labeled, often enough by media or even the wrong head coach and the label sticks.
Heres some aspects that suggest other than head case;
Kostin is 23 and has already agreed to join the fight game to try to find inroads on getting games and minutes. Generally anybody dropping gloves, its considered going the extra mile. Most players don't. The ones that do are usually the ones that will do anything to be here.
Kostin also uses his large frame to hit people. At age 23 the player has already learned to use any edge and attribute to gain advantage. He also uses his size to push off people and get shots off. I like that.
The player also is a solid skater and from looking at it has decent shot mechanics and is NHL level in this regard. Maybe the player is correct in the feeling that he hasn't been given his due. Seems like the player was bringing it. Very few players, only elites find production at the NHL level in their first half of a season. It takes time.
Lastly a 23yr old first round pick in the best league in the world is probably not a head case. One thing or another may have delayed their progress and often its a case of one team not requiring their specific skillset and simply going in other directions.
