In the first five lines it describes the rainy weather outside. The wind picks up as well, and this sets the tone for the poem. The next 10 lines describe her coming inside and warming by the fire. It also shows that she's soaked and cold from the weather outside. She then calls to him and he won't reply. She takes his hand and wraps it around her waist and had his head placed on her bare shoulder, telling him quietly how much she loved him.
The next few lines explain his perception of her--which is very different from what we expected. He describes her as "too weak, for all her heart's endeavor to set its struggling passion free from pride" as if she cannot handle loving him and cannot completely commit to only him.
Once he knew at this point that she "worshipped" him, he grabbed her hair (lines 38-41) and strangled her with it. Even more disturbing is his reaction to that: he opens her eyes and kisses her cheek, then props her head on his shoulder and sits there with her through the night. He does this thinking that it was the right thing to do since God did and said nothing about his actions.
So he did this to preserve the special love that they had forever. He knew it would never last, so he took her life so that their "moment' would be theirs forever.