They're both bad guys, but Pate is just trickier about his evil intentions than Creighton. Pate makes you think he's a stand-up guy because he warns you about a couple of traps the first two times you encounter him. What a great guy, right? But he'll give you things that put you in harm's way, like his ring and armor, which makes you either A) Look like him, which implies he hopes Creighton will kill you instead, or B) Look like you're a friend of his, which once again puts you into conflict with Creighton.
And at the end of it all, if you side with Pate, he shows his true colors. He's no saint, he just manipulates you into helping him deal with Creighton and then once he's done with you, he sends you off to the trapped chest to die.