He's the dude responsible for this whole mess, really, since he helped Voldemort come back to life back in the fourth movie. And now he has to slink around in terror and try to keep Voldemort happy—not an easy job.
When the Muggle Studies professor Voldemort has suspended above the dining table at Malfoy Manor starts to groan, for example, Voldemort expects quick action from his most loyal (read: sniveling) servant:
VOLDEMORT: Wormtail! Have I not spoken to you about keeping our guest quiet?
WORMTAIL: Yes, my lord. Right away, my lord.
It doesn't seem like a very happy life, but he's definitely uber loyal to the Dark Lord.