Ultima V combat will work like this...
Enemies normally have 3 choices: attack, move toward player, or pass. As the level of the player increases, the chance for the enemy to pass decreases. If the enemy has a "distance" weapon (like a throwing axe) his chance for moving decreases.
Every enemy has a "flee" flag. If you are kicking the monster's butt, he may decide to escape. This is the only time he will ever move away from the player. Once he decides to flee, he will never pass, attack, or move toward the player again. If you want the treasure, you better kill him before he flees. However, if he is a "non-evil" monster... you better let him go because if you kill a fleeing non-evil monster your virtue will drop.