That's Grima Wormtongue to be precise. Grima Wormtongue… let that name linger in your mouth. How does it taste? Greasy? Sweaty? Like curdled milk on a hot summer day?
Okay, you get the point; just based on his name alone we can tell that Wormtongue is one slimy dude.
But what makes him so slimy (other than looking like the love child of Severus Snape and Peter Pettigrew)? Well, he's manipulating the mind of the enfeebled Théoden, who's under Saruman's spell, he's created some sort of gang to protect him and to unjustly dispose of Éomer when he resists his authority, and he's super creepin' on Éowyn.
But what happens when he's exposed? He immediately cowers in fear, begging for his life, pleading that he was only serving Théoden and that he did nothing wrong. The dude's groveling on his knees the moment he squirms from Gimli's grasp. Théoden calls Wormtongue's magic leechcraft, and he is certainly a leech—he hangs on to whoever is in power.
Keep this tendency mind after Isengard falls. Do you think Wormtongue will continue to serve the disposed Saruman?