Regarding number 2 (tehehe - sorry immature sometimes), this game is a little different from most in that hitting someone else's mob can actually give you experience. You have to do a certain amount of damage to the mob to get credit, but it is generally in your benefit to "help" ESPECIALLY if you are doing the same quest. Mobs can be a little scarce on some quests so if I notice someone killing the same mobs as me in an area here is what I generally do.

1. If I am the one to tag the mob (it is red to me), I do just enough damage to ensure I have hit it enough and then just leave it alone. This ensures that the other person hitting the mob is also able to get enough hits in to get credit.

2. If I am not the one to tag the mob first I will simply do enough damage to get credit and then leave it alone, again to ensure that anyone else around has ample opportunity to get credit for the kill as well.