If I oppress people I am opressing myself. The way I see people, I get this sense that they are not too different from me. Yes, they are impossibly different, yet they're is something that connects people... call that collective conciousness or whatever you want. I feel like there is some kind of amazing thing in every person, so killing someone or opressing a group would be difficult.
The exception is when a group is acting to oppress another group without justification. For example, criminals are a group that are justifiably oppressed, while nazis are a group that does not justifialy oppress. I would have no problem oppressing nazis (that is, if they were acting to oppress... I support teir freedom of expression, but do not support their actions or beleifs), and I only want criminals to be oppressed in a fair way that encourages change and productivity.
I don't know what all this "modernist" and "post modernist" suff is, so call it what you want.
So... oppresion is wrong to me because it is limiting the abilities of people who deserve to grow and flourish.