Voldemort got mad because he knew that Bella was one of his most trusted Death Eaters... He probably got pissde because he knew she would have done anything he told her too, no matter what it was, and there weren't any others who he could trust to do that. Voldemort, as we all know, liked wasting minimum effort on things (hence the followers doing his dirty work, the use of unforgivable curses which made controlling, torturing, and killing people all the easier.) True, he can use the Imperius curse, but why waste the effort when he can just tell Bella to do it, right? He lost that when she died.
And I agree with Orna. He is, after all, a man. Evil, creepy, twisted, and sadistic, but then, so are rapists, and lust is all they really feel. He could have been using her for sex, for all we know. She would do whatever he said, so it wouldn't be to hard to convince her to do anything with him. Especially since she was in love with him.