Passion and slaughter, ruth, decay Descend, minutely whispering down, (21-22)
You wouldn't expect a list of all the bad stuff to begin with passion, but think about how passive most of this poem is, and it's not such a surprise. Human agency and emotion may not directly lead to death, but they are associated with certain actions ("slaughter") and reactions ("ruth," or grief), and subject to natural effects of death ("decay"). These all become a foundation for the awed or stunned silence of these lovers.
Our feet once wrought the hollow street With echo when the lamps were dead. (29-30)
The mortal imagery extends to describe the street (hollow) and the lamps (dead).These comparisons are to convey a starkness and foreboding, but they also seem to say that the human condition carries its own death sentence.
I do not love you less that now The caged heart makes iron stroke, (33-34)
Love is not lessened due to life's limit; on the contrary, you probably have to stay in the moment to, you know, carpe diem. The image of the heart in its cage acting like a machine in its last heavy motions is chilling. You can practically feel the effort, count each beat. Yeah, you're going to die, but you can't let that crush you.
We live in time so little time And we learn all so painfully, That we may spare this hour's term To practice for eternity. (37-40)
By now, the speaker sounds resigned to his mortality, able to apply a balanced poetic logic in defense of his inactivity. You can cheat death's finality by finding moments, like this one, which has its own connection to eternity.