In "The Cask of Amontillado," several times Montresor asks Fortunato to turn back. Why would he do this when he is deliberately plotting his murder?

You have asked a great question that to me highlights the
criminal genius of Montresor in his deliberate plotting and leading on of Fortunato to
his death. Montresor is trying to make himself sound reasonable and to not let Fortunato
suspect any foul deeds are in the offing, so he deliberately tries to appear to persuade
Fortunato to turn back. However, if we examine what he says closely, we know that
Montresor knows the weakness of his enemy and is using it against him to lead him on to
his doom.

Let us consider what Montresor tells us about
Fortunato:

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 connoisseurship in
wine.

It is this fatal
weakness, this Achilles heel, that Montresor exploits. Note how he uses the Amontillado
as bait to lure Fortunato down. What he cunningly does is to suggest that he should go
to Luchesi, another noble, instead of Fortunato, as Fortunato is obviously so busy. Of
course, suggesting that somebody else give information on what is Fortunato's area of
expertise only strengthens Fortunato's determination to go with Montresor. Any further
doubts that Montresor expresses are clear to use the same
strategy:

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"Come," I said, with decision, "we will go back;
your health is precious. You are rich, respected, admired, beloved; you are happy, as
once I was. You are a man to be missed. For me it is no matter. We will go back; you
will be ill, and I cannot be responsible. Besides, there is Luchesi
-"

Note the final four words.
You see, part of Montresor's cunning is that, whilst ostensibly being concerned for his
"friend's" health, he is actually showing that concern in a way which can only tempt
Fortunato to go further into the catacombs with Montresor to his
doom.