"The pair, who are also French..." Right, these guys are French like a horse in water is a fish.
France has had people of multiple origins since the early 1700s or arguably even earlier.
As to the Algerian genes of both these murderers, Sabrina is part Franco-Algerian and Ouissem seems wholly of Franco-Algerian origin -- and these families date from the time when the French government declared Algeria as part of France, a French department (state) -- so no more strange than that there are hawaiian names among US citizens, or a Ted Cruz can be a politician.