This famous quote comes from Act V, Scene 2 of Shakespeare's Othello. The line is spoken by Othello as he enters his wife Desdemona's bed chamber, intending to murder her for suspected infidelity. The line refers both to the putting out of a candle, and to the ending of Desdemona's life. Here, Lestat appropriately quotes it as he knocks over a candle, extinguishing the flame, and prepares to murder a woman in his hotel room.
Put out the light, and then put out the light:
If I quench thee, thou flaming minister,
I can again thy former light restore
A much-quoted phrase from the Bible, usually considered to mean that, as humans, our vision of events is often obscured, as if we were observing them in a darkened mirror. When Lestat tells Louis: "You no longer look 'through a glass darkly,'" he is telling Louis that he cannot return to the mortal world with his newly acquired knowledge: Louis can no longer experience mortal life. His glass is no longer dark.
When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known. And now abideth faith, hope, charity, these three; but the greatest of these is charity.
1 Corinthians 13:11-13
The story of Endymion comes from Greek mythology. There are various accounts of his story. Perhaps the most famous describes Endymion as a shepherd of such unsurpassed beauty that Selene, the goddess of the moon, fell in love with him. She begged Zeus to keep Endymion as he was, so his beauty would not fade. Some accounts suggest she loved the way that he looked when he was sleeping so much that she asked Zeus to keep him in that state. Yet another account has Endymion himself choosing to sleep forever. The end of most accounts is the same, however: Endymion sleeps eternally, beautiful and perfect.
Lestat is referring to the fact that, having preserved Endymion in all his beauty, Selene can never have him as he is permanently asleep. Her gazing on him is as futile as Louis watching Babette: he can never do anything but look at her.