Finally this is drifting to something a little interesting again! What did happen when the arabs invaded byzantine North Africa? That area seems to have descended into backwater status around the time.
Lot of debate about that. The eminent Walter Kaegi has recently published an overview work on the invasion - like, in the last year - but his interpretation has not exactly met with universal approval. You should check it out - preferably by borrowing it from a library, of course, because it's freaking expensive.
Karchedon was certainly not reduced to "a small garrison fort and a few pathetic squatters living amongst grandiose ruins" by the late seventh century. While the region had certainly declined from its high point of prosperity in the late fourth century due to the collapse of the trade spine with Rome, it was not a
precipitous decline. ARS (African Red Slip pottery, made chiefly in Karchedon and the environs) was still produced, and still appeared widely in the Mediterranean up to about the same time as the Muslim conquest. We've tended to track the decline of Mediterranean trade chiefly through the decreasing amounts of ARS visible in period archaeological sites; the disappearance of such pottery from all but coastal Iberia, for instance, in the fifth and sixth centuries, is a good barometer of how disconnected the peninsula was becoming from the remnant of the Mediterranean trading economy. Anyway, point is that ARS was still being made in what appear to have been significant numbers through the seventh century.
In terms of population, well, we're not really sure, but as you know we're never really all that sure about populations in the relevant period anyway. There's clear signs of habitation throughout most of the city. North African political life was relatively vibrant and there was a great deal of discourse about religious controversy with the central authorities in Constantinople. The result of the Battle of Yarmouk, for instance, was frequently connected to the Emperors' meddling with monotheletism and monoenergism, both of which were regarded as pretty much heretical. Konstas II's promulgation of the
Type in particular seems to have ruffled a lot of feathers and instigated some persecution.
Africa was still valuable to anybody who would choose to hold it. A century before - well, slightly less - Herakleios and his father had waged a civil war from Africa against the central authorities and won; somewhat later, the exarchos Yennadios had attempted to resist Konstas II and held his own for quite some time before the Emperor moved west, established his residence at Syrakousai, and forced Africa's submission.
Nevertheless, Africa's
military resources were somewhat scarce, made slightly more so by the region's low overall population and recruitment base. They also had low renewability, forcing the exarchal authorities to rely on recruits from Mauri and other tribal groups to make up losses suffered in battle. Reinforcements from Anatolia and the Aegean were effectively out of the question, especially considering the Byzantine fleet's parlous situation in the second half of the seventh century, weakened by defeats like the Battle of the Masts. It was considerably isolated from the rest of the Byzantine state. Even if it had not been, the region was rapidly becoming a hotbed of opposition to certain key planks of imperial policy, planks that would not really change until after the Seven Revolutions. It may have been deemed too much trouble to save.
With all that in mind, it is easy to understand the collapse of Byzantine power in the region. An initial battle permitted the Muslims to establish a foothold in the region at Qayrawan, which badly destabilized what was left of Byzantine government and drove a wedge between the Mauri and the imperial authorities; by the late seventh century, the ultimate military result just had to be played out.
On the subject of atrocities: I wasn't aware that the capture of Karchedon was particularly noteworthy one way or the other. I haven't read anything about massacres outside the norm during the conquest of Byzantine Africa, and neither have I read anything about unusually clement behavior on the part of the conquerors.