If a player does not role play his or her career, is a GM within their rights to mandate a career change? Or if it fits the story? Say for instance a dwarf character epically fails at a key task, it mike make sense in the scope of the story for him to be mandated into a slayer path. Or is this taking on too much authority as a GM?

GM has full control of what goes on, but you have to consider how the players will take your heavy handed nature. You might just upset the group with such rulings when it probably is best to try and be diplomatic about it and influence a person to go along with your "ideas".

It's always best to give the players an incentive to do such things by their own initiative. Otherwise the GM will become the villain and the game will suffer greatly.

A GM might veto paths which he felt were incompatible with the character.

Otherwise, no. If you have a 'bad' roleplayer then try to talk to them about the situation. Try to resolve it amicably.

In the end you might need to learn to live with it or tell the player he needs to find another game.

King Tycho said:

As a GM it's within your realm to suggest that becoming a Troll Slayer may be a logical step to take. It's not your position to mechanically force them to do so. Ironically it's well within your position to prevent someone from taking a certain class if it doesn't fit.

King Tycho said:

There are different ways to resolve the issue. If the Dwarf had taken an oath he can no longer fulfill, or he failed at this oath, then it does make sense for him to become a Slayer. I'd approach the player and suggest that this is the case, and that he should decide if he wants to become a Slayer or not. If he doesn't become a Slayer, he should understand that he will be treated as an Oathbreaker by other Dwarfs.

This is really no different than any cause and effect situation. If someone plays a character who steals a horse, then he's likely to become a wanted man.