The "temple-builders" were a Neolithic group that arrived in Malta around the 5th-4th millennium BCE. They were a successful Neolithic society, possibly related to the Sicani of Sicily. A successful Neolithic society is typically marked by advancements in pottery development/use, cultivation of crops like cereals, artistic development (especially figurines), and building projects, often megalithic, usually as the result of social stratification in some form or another. Malta is just one example of this type of "successful" Neolithic culture, though the structures created at Malta are quite distinct. Europe and the Near East is dotted by various Neolithic societies at this time, all more or less "succeeding" in these basic ways. We find others in the archaeological record which did not "succeed." Why or why not is a complicated question.

No one knows what happened to the temple-builders at Malta. Like all Neolithic societies, they were eventually supplanted by Bronze Age peoples, though not necessarily by invasion or violence. It was once thought that the end was abrupt, and came about 2400 BCE. Now most think that the abruptness of the break is due to our severe lack of general evidence and clear stratigraphy at many of the sites. It is very probable that the Neolithic society disintegrated over a rather longer span of time, due to factors beyond our understanding. It could have been related to climate change, natural disaster(s), mismanagement of resources, societal upheaval, or any combination of such. We can see in the late Tarxien phase (the last of the megalithic builders) that changes are happening in how land was utilized, with more marginal, less productive land coming under increased cultivation (that is the general impression from archaeological surveys). This could point to societal fracturing, poor harvest due to climate change, or even overpopulation due to mismanagement of island resources or the arrival of a new, populous group. There are no definitive answers at this time.

The most famous scholar of this topic is Luigi Ugolini, who was determined to prove the Neolithic dating of the megalithic structures (vs later, Bronze Age date). He died untimely in the early 20th century, but a dossier of his work and planned publications is currently under restoration and publication. He possessed many drawings of the megalithic structures which preserve elements no longer extant or visible, including many of the delicate artistic renderings which have, since excavation in the 19th century, weathered away to nothing. A good modern source for this is Claudia Segona, The Archaeology of Malta (Cambridge 2015). In the same year, Oxford put out a collection of studies called The late prehistory of Malta : essays on Borġ in-Nadur and other sites.

I wrote this post about the religious beliefs of the temple builders. It also has some general information about them.