This question has been grappled with since time immemorial. There have been numerous interesting metaphysical explanations; Sri Anirvan's point of view (see Buddhi & Buddhiyoga in Inner Yoga) is telling, as is Ibn al Arabi's. Taken together the two present what may be some of the most sophisticated, and fascinating, points of view on the matter—for those who can absorb philosophy, that is.
Let's try to put it in some simpler terms.
We might say that all creation falls away from God's Grace; some of it falls further away than other parts; and indeed this is exactly how Swedenborg describes the nature of things, that is, in terms of their proximity to God. All faces ought to be turned towards God; to the extent that they turn away, so they fall further, for to turn towards God is to be drawn inexorably towards God; all of creation's fondest and inexpressible initial wish is to turn towards God and draw closer to Him.
Paradoxically, that wish loses its force in proportion to distance; so if one falls away, the wish becomes weaker, and the impulses that the wish engender become lower as well. God is a movement towards unity, which is how Meister Eckhart describes essence in sermon 36; so that movement also encompasses relationship and togetherness. To the extent that we fall away, we fall further and further apart from one another; and from ourselves. So the movement away from God is a movement into selfishness: and because personality, in its coarsest and least intelligent manifestation, is drawn outward and thus further and further from that central good from which it emanates, so it is more and more likely to act selfishly, that is, for itself alone and against the common good.
From this action, what we call bad arises. Bad doesn't really exist in itself; all it really is is a measurement of distance from the good, but as things grow more distant from the good they are in more and more pain (even though the bad perversely experiences pain as pleasure) and more and more desperation (which engenders an ever stronger and more willful ego), so they act in more and more destructive ways.
This kind of action ought to be viewed with great sadness. Yet we are impacted by the bad and it distresses us; we need to muster strong forces inside ourselves to resist. Thus Christ said: Greater love hath no man than this, that he lay down his life for his friends. (John 15:13.)
To lay down our love for our friends is to offer ourselves to the great good, despite our temptation to do the opposite. To go against what is selfish is to love.
This is the melting outwards. It involves taking all of the loving Being we have within from our relationship with the Lord, and offering it outward as unconditionally as possible, without judgment. Now, of course, that is very nearly impossible; we fail a thousand times a day. Yet, as Michel Conge says:
When I move toward a more authentic attitude in myself and want to put it to the test of an encounter in life, and especially an encounter with another being ... in a flash, everything is destroyed! Nevertheless; that's what we'll have to experience, little by little. What does it matter if I fail every time! I have to come back to it, I must go toward it, I mustn't let myself be discouraged or stopped by the fact that it's almost impossible. It will be almost impossible thousands of times ... then, suddenly, perhaps something will become possible. I don't know on what day, at what moment, or even why.
If I don't attempt the experiment, I do not allow the condition that is necessary for this latent possibility to be fulfilled. This form of work is very painful ... and yet, it's impossible to escape. I must go through that: put something to the test, expose it, risk it. And risk it knowing that I will lose every time! —Michel Conge, Inner Octaves, P. 98
—even if only by an inch.