Read "Love Malevolent" at The Eldritch Dark:
This is a dark little number from Clark Ashton Smith (CAS). In general, one could say that the theme of the evil lover is a tried trope, and I can't say that CAS does anything remarkable with the idea in these lines.
This sonnet does have many echoes of "Duality", which I read yesterday. Where that poem's imagery was lush and hypnotic, "Love Malevolent" uses some of the same elements (poisonous plants, snakes, a tomb) in a more explicitly gothic manner, and does so in the service of the rather simple notion of a dangerous lover.
Interestingly, CAS chose not to include this poem in his omnibus Selected Poems (1971), which makes me think that the author recognized it as not being one of his best works.