And Er, Judah's firstborn, was wicked in the sight of the Lord; and the Lord slew him.
-- Gen. 38:7 (KJV)
In this verse, Er was struck dead by God for being wicked. The text does not explain what Er did to cause for the Lord to strike him dead. The implied reason could be in the meaning of his name. Er means "watchful." When a person does not choose to fulfill the calling within one's name, then that person will be rendered useless upon this earth. In the case of Er, he must have chosen to not remain watchful and alert to the voice of God.
Another theory is by looking at the context of the verse. The beginning of this chapter so far has been themed around child-bearing. From the context, Er must have refused to have sex with his wife.
Numerical Analysis: The numerical pattern for this verse is 2,7,9,3,8. (2) To be delivered, remain alert. (7) Be perfected by God in order to bear fruit. (9) To overturn evil, do not remain wicked in the sight of God. (3) Stay in the Spirit or perish immediately. (8) Choose to have a new beginning in God or perish suddenly.