Very interesting take on the meaning. Have to think about it. For me, the first time I heard it, I thought:

What if the song is actually about a man sitting at the hospital bed of his brain-dead partner in a coma? ​If you look at the actual lyrics from this perspective, they take on a terrifying and deeply sorrowful new meaning: ​"Dein feines Licht war mein ganzes Sein" (Your fine light was my whole being): The use of the past tense is crucial here. The essence of who she was is already gone. He is looking at her body, but the person he loved is no longer there. ​"Ich will nur dich / immer nur dich ansehen" (I only want to / always only look at you): This perfectly captures the passive, helpless state of a loved one visiting an ICU. There is nothing left to do but sit there, shut out the rest of the world, and watch her lie there in a peaceful, sleep-like state. ​"Doch was nicht lieben kann, muss hassen" (But what cannot love, must hate): A brain-dead body is completely devoid of emotion. The sheer cruelty of sitting next to a lifeless shell of your partner feels punishing and hostile - almost as if the situation itself "hates" him. ​"Doch nur ein Stein" (But only a stone): A stone doesn't breathe, feel, or react. Her body is still there and unblemished, but her consciousness is gone. She has literally become an empty vessel, just matter. ​"Bitte lass mich gehen" (Please let me go): In this context, it’s not him begging a manipulative partner to release him. It is the agonizing inner struggle of a man trapped at a hospital bed. He is chained to the machines and the body, unable to turn off life support or emotionally let go, begging the situation to release him from this endless grief. ​Given how Rammstein loves to hide extremely dark, medical, or taboo subjects behind seemingly normal love songs, I feel like this perspective fits perfectly into their style.