The Question and Answer section for Hamlet is a great resource to ask questions, find answers, and discuss the novel.
In my opinion, Polonius' actions certainly mark him as devious, but his willingness to leave his daughter alone with someone he believes to be mad seems almost evil.
Polonius' suggestion that they spy on Hamlet infers that he is sneaky and not above reproach.
From their interaction, we can see that the two men are well-liked by Hamlet, but that they're also unepected visitors. It's almost as if they're in the way...... an inconvenience.