Why did Montressor decide to kill Fortunato in his own basement instead of somewhere else?
Montresor did not kill Fortunato in his "basement" although that's the feel we get from their journey toward it. Montresor chose to murder Fortunato in the the catacombs because of its seclusion from the outside world and the likelihood of someone suspecting a murder took place was slim to none if anyone found Montresor's body. Catacombs were underground tombs for an estate's family so finding a buried body among the walls would be nothing out of the ordinary. Not only is he virtually burying a body in a cemetery, but he there is also little chance that Fortunato's screams would be heard above because Montresor has taken him to the lowest point in the catacombs and his house has been emptied for the night. It was the best place to bury a person alive, much more so than any other place.
The murder of Fortunato took place, not in Montressor's basement, but in the catacombs, family crypts, of Montressor's family. This site was perfect for his crime for several reasons; the first of these is that it was his family crypt. I don't know about you, but to be perfectly honest, I can't think of anyone who just decides to take a walk in their family crypt! Therefore, it is not likely that anyone would stumble upon the murder scene; another benefit is that Montresor himself has access and can keep an eye on the scene whenever needed. Finally, there is the point to be made that no one could hear Fortunato when he screams, which I suspect he did for as long as he had the strength. These are the basic reasons that Montressor chose to kill his enemy in his family crypt.
Montressor killed Fortunato in his basement (also known as the catacombs) because the catacombs were used for keeping dead bodies in it, and also for storing the wine, so Montressor killed Fortunato in a place where dead bodies were kept, therefore, if Montressor put fortunato's dead body in the catacombs, and some people found his body, they would think it was just another dead body.