Is a person considered an accomplice to sin and aggression, and sinful, if a website linked to their Twitter account retweets thousands of tweets sent to specific accounts, some of which contain insults and defamation, due to their linking their account to this website?

It appears you would not be aiding in sin and aggression, as a person is not sinful for creating things that can be used for both permissible and forbidden purposes, unless they know or strongly believe that they will be used for forbidden purposes. Offensive tweets should be deleted if possible.