Obviously Malfoy was sent to kill Dumbledore. But wouldn't it just have been easier to kill Harry? Why did Voldemort only have him kill Dumbledore, when there were countless opportunities to do them both off at separate times?
Malfoy was probably not instructed to kill Harry for these reasons: Revenge, and pride. Voldemort has shown a history of personally taking care of those who have offended him. Not to mention Harry has been the only person to survive the killing curse... Yeah. lets just stick with pride as the reason Malfoy wasn't instructed to kill Potter.
There may be several factors at play here.
Firstly, it seems that Voldemort was not actually trying to use Malfoy to accomplish a goal. Rather, he was punishing the Malfoys by giving Draco an impossible task that would probably result in his death. As Narcissa states in Chapter Two of Half-Blood Prince:
"Then I am right, he has chosen Draco in revenge!" choked Narcissa. "He does not mean him to succeed, he wants him to be killed trying!"
And as Snape and Dumbledore discuss in Chapter Thirty-Three of Deathly Hallows:
Scowling, Snape said, "The Dark Lord does not expect Draco to succeed. This is merely punishment for Lucius’s recent failures. Slow torture for Draco’s parents, while they watch him fail and pay the price."
"In short, the boy has had a death sentence pronounced upon him as surely as I have," said Dumbledore.
This being the case, Voldemort would not have given Draco a task that he could easily succeed at, e.g. killing Harry.
Additionally, at this point Voldemort was still uncertain about issues surrounding Harry. Draco was given his assignment in the summer between Harry's fifth and sixth years. That is around the same time that Ollivander was kidnapped. Voldemort had just failed to hear the prophecy, and didn't quite understand the whole wand situation. He was trying to extract information from Ollivander that would help him understand how to defeat Harry.
Thus, it is likely that he would be hesitant to plan anything regarding Harry at this point, until he could get a better idea of what the issues were and what needed to be done. Note Voldemort's statement in Chapter One of Deathly Hallows:
"I have been careless, and so have been thwarted by luck and chance, those wreckers of all but the best-laid plans. But I know better now. I understand those things that I did not understand before. I must be the one to kill Harry Potter, and I shall be."
This was a full year after Draco had been given his assignment, implying that Voldemort only resolved some pertinent issues well after giving Draco his orders. Indeed, Voldemort was oddly quiet in Half-Blood Prince (at least from what's included in the book), which would make sense if he was somewhat unsure of things and was biding his time waiting for better information.
Thus, Voldemort might not have told Draco to kill Harry because he wanted to first gain a better understanding before the next attempt on Harry's life.
Furthermore, in Chapter Nine of Deathly Hallows we find another way that Voldemort uses Draco as a punishment:
"More, Rowle, or shall we end it and feed you to Nagini? Lord Voldemort is not sure that he will forgive this time.... You called me back for this, to tell me that Harry Potter has escaped again? Draco, give Rowle another taste of our displeasure.... Do it, or feel my wrath yourself!"
Malfoy’s gaunt, petrified face seemed burned on the inside of his eyes. Harry felt sickened by what he had seen, by the use to which Draco was now being put by Voldemort.
It seems that Draco is hesitant to torture someone that he has no personal animus towards. Voldemort deliberately forces him to do this precisely because it is something Draco does not want to do. Now consider that Draco would probably not have hated Dumbledore. Sure, he may not have respected Dumbledore as much as other students did, he may have thought that Dumbledore had some strange views, he may have thought that Dumbledore favored Gryffindor (and Harry in particular), etc. but none of those are things that would be likely to cause him to want to kill Dumbledore.
Thus, Voldemort orders Draco to kill Dumbledore because that would probably be something hard for him to do. Not just hard physically, but hard emotionally/psychologically. Indeed, when the final confrontation occurs at the top of the tower, we find that Draco cannot bring himself to actually kill Dumbledore.
Contrast this with Harry. It is no secret that Draco and Harry are arch-nemeses. There have probably been plenty of times when Draco wished he could have killed Harry. Of course he never did so, probably in part because he couldn't get away with it. From Voldemort's perspective, ordering Draco to kill Harry might be more like allowing Draco to kill Harry. It would be putting the full might of the Dark Side behind Draco. So Voldemort may have assumed (correctly or incorrectly) that this would be something that Draco would want to do. That might make it more of a reward than a punishment, which would run counter to Voldemort's goals.
Now the question here seems to allude to two possibilities, namely, that Voldemort could have told Draco to kill Harry or that Draco could have simply decided to kill Harry on his own. The above points have addressed the first possibility. As for the second possibility, it is possible that Draco wouldn't dare to do something like that which might contravene Voldemort's orders. Consider his exchange with Crabbe in Chapter Thirty-One of Deathly Hallows:
"STOP!" Malfoy shouted at Crabbe, his voice echoing through the enormous room. "The Dark Lord wants him alive –"
"So? I’m not killing him, am I?" yelled Crabbe, throwing off Malfoy’s restraining arm. "But if I can, I will, the Dark Lord wants him dead anyway, what’s the diff – ?"
Here we see that whereas Crabbe was willing to play around somewhat with Voldemort's orders, Draco was insistent on following them exactly. While we don't know if Voldemort had specifically told him not to kill Harry during his attempt on Dumbledore, it is possible that Draco would simply not have done anything more or less than exactly what Voldemort ordered him to do.