There's no real expediency to killing her off -- Rowling killed them off for a reason. Lupin, in going to war, was doing what men have always done.
Tonks is essentially a soldier, too -- and her choice is more complicated. Does she stay home with her kid, relying on Lupin and everybody else? Or does she, too, fight, so that even if she dies, her child will grow up free from Voldemort's control -- either that, or die trying?
That's a very tough choice to make, and Rowling sets out the worst possible result. The question we have to answer is: did Tonks choose right? Why or why not?
I'd say she did. She IS an Auror, after all.
Women in our armed services have to make the same hard decision.
I think this is another instance of coming full circle. James and Lily died (directly) protecting Harry. Tonks and Lupin were also fighting to protect their boy, but less directly. Both losses occur during a major Voldemort battle. Both children lose their parents. One at the beginning. And one at the end.