To the best of my knowledge there is no way to attack a non-hostile NPC. The mini-map is the best way to determine who is and who is not hostile. All hostiles will show as a red dot with all peaceful NPC showing up as a green dot.
I believe you could have turned the Orcs hostile prior to accepting the quest for the slasher but . . . I always accept the quest and have not tried to make them hostile. At that stage of the game you would quickly be killed by five Orcs. . . =)
The one key that does come close is holding the CTRL while being attacked by a group of hostiles. As long as you have the CTRL key pressed your character will automatically attack the closest enemy until they are wiped out or he dies. The draw back is you cannot drink potions while the CTRL key is pressed and you cannot run away. Learned that the hard way . . . =)