Unethical and probably illegal. You just admitted to logging into someone else's account without their permission. At the very least you probably violated Twitter's terms.
If I never signed up for a twitter account and only signed into one for the first time using these leaked passwords, I haven't agreed to any terms of service.
I have a startup idea, show plaintext username and password on my website for every website i registered, and if somebody logged in using them sue them for money. I will give lawyer 50%. profit and vacation. :D
Unethical and probably illegal. You just admitted to logging into someone else's account without their permission. At the very least you probably violated Twitter's terms.