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This blog post says that they were captured by Cao Cao and escorted back to his territories like Xiangyang and Nanyang, but doesn't give a source. Killing them is unlikely - Cao Cao was too busy fighting and such an event would have been recorded - and following Liu Bei even less so, since this contradicts official history, which says they were "captured".
曹公大獲其人衆輜重。
Cao Cao captured Liu Bei's civilians and supplies.
(Records of the Three Kingdoms)
Civilians have strategic value; when put to productive use, they can pay taxes and grow food. During the Late Han and Three Kingdoms era, there were multiple instances of relocation of civilians for this purpose: both Cao Cao and Sun Quan relocated civilians north of the Yangtze to avoid being captured by the other, Zhuge Liang also did this during his Northern Expeditions.
As for why 100,000 civilians would follow Liu Bei, apart from coercion, they could also be fleeing war, or that they genuinely wanted to follow Liu Bei. During his time at Xinye, Liu Bei was a capable administrator and gained many followers from Jingzhou:
荊州豪傑歸先主者日益多
For the record, the "100,000" figure is most likely approximate but unlikely to be an exaggeration. It comes from Records of the Three Kingdoms, the best source we have for the period and it isn't known for exaggeration:
琮左右及荊州人多歸先主。比到當陽,衆十餘萬,輜重數千兩,日行十餘里,別遣關羽乘船數百艘,使會江陵。
Many of Liu Cong and Jingzhou's people followed Liu Bei. At Dangyang, there was a crowd of more than 100,000, supplies of thousands of carts(?); they travelled more than 10 li a day; Guan Yu was ordered to travel a different route with hundreds of ships, and to meet at Jiangling.