In the first episode at King Arthur's castle, you must bear in mind that no one knows what is going on. Like Grendel, the Green Knight is a sort of creature never seen before by men. Sir Gawain is, throughout the poem, marked by absolute courtliness, that is to say, the is always courageous, honorable (he keeps his word), devout, loyal, and gracious toward all men and (especially) women (pay special attention to the pentangle). He thinks it is his courage that is being tested (wouldn't you?). He does not realize that the incidents in his bedroom at Bercilak's castle, which parallel the hunting scenes (can you guess why?) are actually another kind of test. Thus, when he arrives to face his test at the end, he discovers that he has already been tested--that his test is, in fact, over. Wouldn't you be angry at being tricked this way?
The Green Knight is probably not to be seen as evil. He is a shape-shifter, and Bercilak seems to say that the Bercilak-persona is the "real" one. However, don't give too much credence to his "explanation" of his motivations at the end of the poem (the Morgan le Faye stuff), which is pretty preposterous and doesn't really explain anything.
Ask yourself when you finish the poem: What is really being tested? (This is not a simple question.) How does Sir Gawain do? What are we supposed to think of the Green Knight? Bercilak's wife? King Arthur? his court? Sir Gawain himself?