I may split some of these posts off into an orkish psychology thread in the near future.
There are a lot of things in Tolkien's writings that were either never described definitively, or that were never finalized. A lot of things about orcs fall into one of those categories. Tolkien didn't like orcs, or black-speech, or a number of related issues, and so we don't get a lot of detail in some respects; in others, like their origins, we never get a final version, in spite of the fact that everyone's favourite theory is that they were corrupted elves.
We do know a few things, however.
First, orcs hate and fear. For the average orc, these two things likely rate even above greed. They enjoy hurting things... and will probably do so unless doing so is likely to get them hurt worse. This is, usually, why orcs listen to their leaders; either the leaders are capable of inflicting hurt, or they have followers who are. Killing humans is, thus, something that orcs enjoy. Note, however, that there are elves not far off. Elves are capable of hurting orcs, and killing too many humans will likely make the elves angry... if it doesn't make the humans themselves kill them. So though they wish to do so, they will, likely, be very careful about it.
Orcs, however, wouldn't see themselves as evil; in fact, an orc probably wouldn't have a concept of good or evil, or even a concept of morals.
This might be true, or it might not. Orcs were, however, hard-hearted in the sense in which Marlowe's Faust was. "My heart is harden'd, I cannot repent..."
That is to say, Tolkien wrote (in one of his letters, I believe) that any orc who asked for forgiveness ought to be forgiven, or at least given a chance to atone, but that it was a moot point, as no orc would ever ask for forgiveness.
This gives us room for both: the self-loathing orc who knows that what he does is wrong, but cannot turn aside from it, and the orc who is purely a sociopathic/psychopathic as described above, the one being unable to repent because it knows no better, the other for any number of reasons... perhaps it doesn't think itself capable of redemption. Perhaps it is simply used to doing evil. Or perhaps you have one of many other options as to why your orc continues to act evilly in spite of this knowledge.
They hate things. They hate people who live free from the pain that it is to be an orc. They hate and fear those that inflict pain upon them. They hate those who look down on them. They hate themselves for not being stronger. As an orc, if you can think of something, you can probably
think of a reason to hate it. But in spite of this, they cling to their miserable existence, each in their own little way... obviously, the powerful warriors and war-leaders express these emotions differently, and in a different balance, than their smaller and more cringing fellows.
That isn't to say that greed doesn't have its place... greed and envy both, as well as whatever else you choose to throw into the mix to make it interesting. But these things? These things are a good place to start.