"...the evilness of death derives from the goodness of the life death takes from us:
This portion is explained by the final segment:
disquiet concerning death is the other face of love for life."
Luper is merely expressing his view that when death is viewed as undesirable or bad, this is largely due to it playing out against circumstances in which life is viewed as good, and that the better something seems, the worse its absence can be (and vice versa).
Consider the person who lies in agony in a hospital bed for years, unable to move, unable to control their most intimate of functions, utterly reliant on others to work their limbs so they don't atrophy. For such a person, the 'goodness' of life has been terribly diminished and, as a result of the reciprocal relationship pointed out by Luper, the idea of death may no longer be deemed as evil at all, but as something to be yearned for.
Now consider the person surrounded by people who love and respect them, a person deeply satisfied by their profession and able to enjoy robust physical health, a generous income and a comfortable home. For such a person; a person for whom life is so good, the prospect of death is likely to seem - if not 'evil' in the religious sense - deeply undesirable; ie, 'bad'. The very opposite of good.
The way we feel about things is often relative in this way. With the good must come the bad and vice versa. Surrounded by enormous wealth and success and good fortune, a person who is relatively unfortunate my feel resentful at the inequality from which they 'suffer'. However, place this same person in identical personal circumstances but amidst an environment in which everyone else is much worse off they they are, and their perspective will likely change from one of pure resentment to one of least relative good fortune.
This relativism impacts many if not all aspects of our lives, because as soon as we recognise one standard or circumstance, there are almost always standards and circumstances against which to compare them.
Love and hate, poverty and wealth, pleasure and pain, health and illness: all exist for us on a spectrum against which we can count ourselves fortunate or unlucky, deserving or undeserving. Crucially, we can leverage this awareness to find gratitude in even the direst of circumstances, for there is rarely any place in which might we find ourselves that could not be a lot worse, or at least is a lot worse, for many, many people less fortunate and no less deserving.