The reason for making Varus into a darkin
Is that riot simply didn't want to waste a darkin slot.
So lets be honest here,with the darkin being sentient weapons it was kinda obvious that the bow of Varus would be retconned into a darkin weapon(which became obvious when Rhaast was released).
However if there are only 5 darkin left,then it would have been a huge waste to have a character in the game that uses a darkin weapon,but who isn't a darkin him-/herself.(or at least has the potential to turn into one like Kayn).
So I think the main motivation for rewriting his story that much was because they wanted to turn Varus into the darkin and to create conflict/differentiate Varus and his hosts,so they gave him the whole "driven by revenge for his lost kind" theme(after all he is the "arrow of vengeance"),while the human(s) that make up his body got a different motivation(saving a loved one). The problem for a lot of people though is that(at least from what we know),there is barely any reason to sympathize with Varus the darkin,as the darkin were appearantly the ones who initiated the war and they all seem to be extremely evil beings that pretty much deserved to be wiped out(heck Varus himself was described as a sadistic killer).