You are absolutely right to say that Montresor wants to kill Fortunato. But he keeps giving him chances to leave precisely because he does want to kill him. It's what they call reverse psychology -- he knows that giving Fortunato those chances to leave will make him much more determined to come further.
If you look at the times he gives Fortunato the chances to leave, he usually says that he'll get Luchesi instead. When he says that, he is playing on Fortunato's pride. He knows that Fortunato will never leave because if he did, Montresor would go get his big rival.
So Montresor keeps giving Fortunato chances to leave, but he does it because he knows that it will actually make Fortunato less likely to leave.
Montressor knows that Fortunato will do virtually anything to imbibe in the supposed bottle of vintage Amontillado that he claims to have purchased. Fortunato's love of spirits is overwhelming.
He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine.
Under the pretense that he must quickly receive a second opinion about the quality of the nonexistent Amontillado, Montressor merely threatens to allow Luchesi to sample it instead of Fortunato, knowing that the man's curiosity will be too strong to pass up this opportunity. But it is simply a lure--a ruse--to keep Fortunato interested. By pretending to go back, Montressor makes Fortunato even more curious: so curious that he is willing to risk his health--and, unknowingly, his life--to make the descent into the catacombs.
Montresor may be using reverse psychology in urging Fortunato to turn back, but he has another reason for doing so. Reverse psychology, so-called, plays upon the natural human tendency to do the opposite of what is recommended. This is especially observable in people who are heavily intoxicated. If told that they have had enough to drink, they will order another one. If told they should let somebody drive them home, they will insist on driving their car home by themselves. Reverse psychology may be hard to understand, but it is certainly easy to observe because it is so common.
However, Montresor is much more concerned about something other than using reverse psychology to lure Fortunato to his death. Montresor is leading the drunken man deep underground into a dark, dank, poisonous atmosphere. He doesn't want Fortunato to become suspicious or alarmed. By constantly telling his victim that they ought to turn back, Montresor is demonstrating that he has no evil intentions. He makes it seem as if he really doesn't want to take Fortunato to the place where he has stored the Amontillado. If he had any sinister motives, why would he keep urging Fortunato to go back upstairs? Montresor has been playing the naive innocent all along. He keeps up the pretence of pure innocence until the last moment.
In "The Cask of Amontillado," one of the conditions that Montesor sets for his revenge is that it "precluded the idea of risk, and that he [the avenger] must not only punish, but punish with impunity." So, if Montesor does not lure Fortunato far enough into the catacombs, there is the risk that the self-professed connaisseur may escape or be heard if he screams.
For the safety of his scheme, Montesor must get Fortunato into the deepest chambers. If he does not distract him by playing upon his jealousy of Luchesi and keep him drunk along the way, Fortunato may realize that he is going deeper and deeper beneath the earth. It is through his playing upon Fortunato's jealousy, his feigned concern, and his supplying of more wine to Fortunato that Montesor is able to get his victim to the recessed tomb designated for his revenge, a spot so far from the opening that no one can hear his screams until he finally dies. (Do not forget that the man is buried alive.)
This is precisely what we call irony. Montresor keeps on insisting that Fortunato should go back, but what he really wants to is to lure Fortunato deeper in the catacombs. It's called reverse psychology.