You mention wanting to avoid a "kill everything" mentality of XP grinding with this system. To me, it seems like the "kill everything" approach isn't so much a result of wanting all the XP as it is the players realizing killing everything is the fastest way to the XP, and there's no in-game benefits to earning the XP another way.
Scenario 1: kill the bugbear. Get the xp.
Scenario 2: spend a lot of time figuring out how to stage an intervention, ask the bugbear if he feels fulfilled by his guard duties. Is he getting paid enough? How are the benefits and advancement prospects. Maybe he would feel more fulfilled by another line of work? Talk him around with some good persuasion rolls and get him to quit his job and go open a bakery. Get the xp. Never see the bugbear again. Realize you just wasted all that time and effort. Resort to killing everything.
Scenario 3: same as scenario 2, but now you have a recurring NPC bugbear baker who might be a good source of information for the players down the road. Also pastries. Payoff - incentive to pursue noncombat options in the future.
Scenario 1: kill the bugbear. Get the xp.
Scenario 2: spend a lot of time figuring out how to stage an intervention, ask the bugbear if he feels fulfilled by his guard duties. Is he getting paid enough? How are the benefits and advancement prospects. Maybe he would feel more fulfilled by another line of work? Talk him around with some good persuasion rolls and get him to quit his job and go open a bakery. Get the xp. Never see the bugbear again. Realize you just wasted all that time and effort. Resort to killing everything.
Scenario 3: same as scenario 2, but now you have a recurring NPC bugbear baker who might be a good source of information for the players down the road. Also pastries. Payoff - incentive to pursue noncombat options in the future.