Relativism CAN exist within a DnD like alignment system, if you have a good enough DM. I'd even go so far as to say it is essential.
Like this:
An Orc is injured and dying. Player X kills the orc as an act of mercy and relief.
Player X is a GOOD player and should recieve good aligment shift
or
An orc is injured and dying. Player Z kills the orc because it might divulge information to its fellows if left alive.
Player Z is NEUTRAL and should recieve a neutral alignment shift.
or
An orc is injured and dying. Player Y kills the orc because they want a trophy to add to their collection.
Player Y is EVIL and should recieve an evil alignment shift.
Same act, three different reasons, three different alignments. Essentially depending on the 'reasons' you can call almost any act good or evil or lawful or chaotic. Absolute morality in a PnP setting is boring and leaves little room for interesting character expansion. Relative morality is not only more realistic it also gives your PCs more room to develop in their own right and in their own way.
