I think Lucy and Willoughby are co-villains in S&S. Lucy does less damage because her scope is narrower. She makes a few people miserable for a time. Willoughby has the advantage in being a man. He roams, has multiple victims and leaves serious damages in his wake. He ruins Eliza’s life and nearly Marianne’s and Sophia may already know she has been had, who knows about the nameless others in his past and let’s not forget he abandoned his child. Willoughby lacks empathy, willingly risks other peoples feelings and welfare to obtain his pleasures and goals as is Lucy but his ill-intent does not seem to rise to active maliciousness. He is resentful and unforgiving of Col Brandon and Sophia. Col Brandon was never charmed by him and held him accountable for his seduction of Eliza while I think Sophia’s crime is in not being Marianne and having outlived her usefulness. Morally Willoughby is the worse villain of the two however I think Lucy exceeds him in one propensity. Lucy is actively malicious and seeks revenge for perceived slights and betrayals. She tortured Elinor with her engagement to Edward long after she had staked her claim on him. I suppose for the crime of catching his eye. She lashed out at Marianne for telling Edward to think of Elinor:

"Perhaps, Miss Marianne," cried Lucy, eager to take some revenge on her, "you think young men never stand upon engagements, if they have no mind to keep them, little as well as great." (Ch. 35)

After she exposed the engagement, Nancy said she “never saw Lucy in such a rage in my life. She vowed at first she would never trim me up a new bonnet, nor do anything else for me again, so long as she lived” (Ch. 38). I think Lucy purposely strands Nancy in London without means to travel. Perhaps as punishment for exposing her plans, being a continual liability for years and a general thorn in her side and maybe because she no longer has any use for her and does not wish to share her good fortune:

Lucy, it seems, borrowed all her money before she went off to be married, on purpose, we suppose, to make a shew with, and poor Nancy had not seven shillings in the world; -- so I was very glad to give her five guineas to take her down to Exeter… (Ch. 49)