Monsters are .. monsters. At a baseline, they do nasty things and are generally not fit for life among more civilized or grey-leaning-towards-light settlements. In fiction, they're usually the footsoldiers or pawns of more sophisticated villains than they for good reason; their base urges can be manipulated by a cunning villain or turned against them by a clever hero.
The monster mercenary in question is fine. It is also fine that he'd want to butt in on big events to get a share of the loot, and get his way by virtue of being a useful and powerful individual. It is also fine that more rigid-minded PCs would refuse to work with him. None of this is a big issue, really. There doesn't need to be a big, ironclad rule allowing or forbidding people from associating with monsters aside from what the player finds in-character for their PC, to me it's far more interesting for there to be diverging opinions and people taking action for or against mingling with monstrosities than everyone be in lockstep whenever an unorthodox situation occurs, which to me feels a bit like where this thread is going.
If your PC has reason to antagonize a monster, they can take action accordingly. The monster has reason to antagonize most anyone too, it's fair game and it should be expected when taking the steps of writing an application for one that justifies being more than a bunch of +4 bonuses on a stats sheet.
For those who are not aware, there is a Forgotten Realms Wiki with extensive lore on most things, from which this server's lore is based upon, and relevant to this particular discussion it has information on the many monstrous races of Age of Netheril:
https://forgottenrealms.fandom.com/wiki/Main_Page