The word "still" is interesting; it can mean both "remaining the same," as in "even after you get old, I will still love you," as well as "motionless" or "quiet" (like a corpse). Not to get all morbid or anything, but it almost sounds like the speaker is saying, "I will luve thee still" in the sense of "I will luve you until you are still and motionless." (Just think of the phrase "I will love you silly.") This interpretation makes the poem seem just a teensy bit darker. Okay, maybe a lot darker.