Say a general was spotted commanding his army from a hill a ways behind, what would the enemy general have thought of the idea of sending a force to capture and/or kill him? Would this have been seen as a legitimate tactical decision, or would it have been considered "dishonourable" or some such?
There is evidence of the attitude among European military leaders, but this didn't stop several notable generals, as well as countless forgotten colonels and officers from being killed. Christopher Duffy provides two examples of the attitude in his Military Experience in the Age of Reason. Before the Battle of Rossbach, the French commander elected not to make an attempt on Frederick the Great (and was swiftly crushed in the ensuing battle), and during the American Revolution, Lafayette showed the same courtesy to two British generals, Philipps and Arnold, refusing to sanction anything like assassination.
During the American Civil War, there was a famous incident following the battle of Fredericksburg, when both armies were in winter quarters. The Union pickets heard from their Confederate counterparts that the officer they spotted accompanied by a party of ladies and staff was none other than Stonewall Jackson, who'd led his command to victory after victory over the Union forces; he was close enough to shoot with a revolver, reported Elisha Hunt Rhodes, but the agreement was that neither side would fire. Outside of battle, it was 'simply murder.'