What was the reason for the change in the Borg, to go from assimilation to killing and mass murder of entire populations?
As explained in Greater Than the Sum
's destruction of the Borg transwarp hub elevated Starfleet/the UFP from "minor nuisance" to "serious threat." Essentially "Endgame" was the Borg's 9/11, shaking them out of their complacency and sending them into high alert against a suddenly major enemy. And given all the other times that Starfleet had successfully resisted assimilation or stymied Borg plans, the Borg recognized that their conventional tactics would not work against the Federation. So they did what they always do: they adapted. Faced with a serious threat they couldn't conquer or tolerate, they chose eradication as the only viable alternative. It took the Borg a few years to find a way to reach the UFP and carry out that plan, though. (The Borg seen in Resistance/Before Dishonor
/GTTS were a separate offshoot of Borg, cut off from the main Collective due to the transwarp hub's destruction, acting on their own. Their "kill rather than assimilate" behavior was a defense mechanism activated by their need to protect their nascent Queen in Resistance
. As for why they continued to enact that approach afterward, I guess that since the Queen's "birth" was aborted, they never got the order to stand down from that hyperaggressive state.)