The original plan was for Draco to succeed so Snape could retain his position, 
or for Draco to fail and for Snape to take over. Either way, it would be a good lesson for the Malfoys. Snape says as much at the beginning of the HBP.

But then, in the same scene, Snape makes the Unbreakable Vow with Bellatrix's assistance, no less. She's practically goads Snape into it.
Which is very intriguing. Bellatrix is Voldemort's most loyal follower, so she probably wouldn't go against her master's wishes just to protect Draco. At the least, she believes her master wishes for Snape to assist Draco and it could be she even does it by direct orders. (She may be jealous and distrustful of Snape, but she gets that her master has a different opinion.)

So things are looking good from Voldemort's perspective at the beginning of the school year. He had chosen Draco for a hardcore mission, teaching Lucius (and other death eaters) what happens if someone fails him. Still, the mission is in good hands, as the boy has the assistance of his beloved potions master, the friend of the family he knows from childhood and trusts. Snape is competent enough to whip up some deadly poison and come up with an effective plan, hopefully one that will allow him to stay in the background so his cover wouldn't be blown so Voldy's got to keep his valuable spy in Hogwarts.

Except Draco refuses to listen to Snape, concluding that his teacher has his own reasons to offer help.

> "I know what you're up to! You want to steal my glory!" (HBP, CH15)

So Draco runs unsupervised and sure enough, he messes up. Twice. Maybe it's good for the pacing, but it probably drove Voldemort crazy. Well, crazier. Draco's assassination attempts are described by Dumbledore as laughable, and with good reason.

> 'Forgive me, Draco, but they have been feeble attempts ... so feeble,
> to be honest, that I wonder whether your heart has been really in
> it...'

And those attempts weren't just feeble, they were painful to watch, a complete disaster.  
Anyone less cavalier about students being in danger and getting hurt than the illustrious headmaster, would've traced back the necklace to B&B, and found the wannabe assassin in record time. As we know, the reason Dumbledore didn't stop Draco is because he chose not to. But from Voldy's POV, Draco is fooling around in the castle, endangering the mission and putting Dumbledore and the Order in alert and risking to blow the cover of his most valuable spy. Good think Voldy had no hair by that point else he'd pull it all out.
And all he could do is to sit back and wait how things will turn out. After all, he himself gave the mission to Draco :)