I've done tons of healing people/casting stoneskin on people who are having trouble in fights, and the only reaction I've ever gotten is people thanking me (/bow or /salute or whatever). I've also helped people by fighting/killing/provoking mobs they seem to be having trouble with, and again, never had anyone upset with me. But if you're going to hit the mob -- and you're just trying to help -- make sure you let the person hit the mob first; that will allow that person to get the claim on the mob, so they'll get the XP and any drops. Otherwise, they might think you're trying to take over their camping spot.

It's an MMO. Interacting with people is one of the best parts. And people tend to appreciate random acts of kindness.