Section 230 of what?
Twitter Terms of Use:
"1. Who May Use the Services
You may use the Services only if you agree to form a binding contract with Twitter and are not a person barred from receiving services under the laws of the applicable jurisdiction....
3. Content on the Services
You are responsible for your use of the Services and for any Content you provide, including compliance with applicable laws, rules, and regulations. You should only provide Content that you are comfortable sharing with others... [This means that you can be held accountable if you retweet something that does not meet the aforementioned conditions.]
... We reserve the right to remove Content that violates the User Agreement, including for example, copyright or trademark violations or other intellectual property misappropriation, impersonation, unlawful conduct, or harassment."
The Twitter Rules:
"Safety
Violence: You may not threaten violence against an individual or a group of people. We also prohibit the glorification of violence. Learn more about our violent threat and glorification of violence policies.
Terrorism/violent extremism: You may not threaten or promote terrorism or violent extremism. Learn more....
... Civic Integrity: You may not use Twitter’s services for the purpose of manipulating or interfering in elections or other civic processes. This includes posting or sharing content that may suppress participation or mislead people about when, where, or how to participate in a civic process. Learn more....
... Synthetic and manipulated media: You may not deceptively share synthetic or manipulated media that are likely to cause harm. In addition, we may label Tweets containing synthetic and manipulated media to help people understand their authenticity and to provide additional context. Learn more...."
Trump violated these rules on numerous occassions. By violating the agreeent, he forfeited his privilege to use their intellectual property; for which, they can terminate his account on an at-will basis due to the agreement that he violated.