In my state, if he opened the door and walked into the dwelling, he is guilty of criminal tresspass.
Sorry, but you are incorrect:
Sec. 30.05. CRIMINAL TRESPASS. (a) A person commits an offense if the person enters or remains on or in property of another, including residential land, agricultural land, a recreational vehicle park, a building, a general residential operation operating as a residential treatment center, or an aircraft or other vehicle,
without effective consent and the person:
(1) had notice that the entry was forbidden; or
(2) received notice to depart but failed to do so.
(b) For purposes of this section:
(1) "Entry" means the intrusion of the entire body.
(2) "Notice" means:
(A) oral or written communication by the owner or someone with apparent authority to act for the owner;
(B) fencing or other enclosure obviously designed to exclude intruders or to contain livestock;
(C) a sign or signs posted on the property or at the entrance to the building, reasonably likely to come to the attention of intruders, indicating that entry is forbidden;
(D) the placement of identifying purple paint marks on trees or posts on the property, provided that the marks are:
(i) vertical lines of not less than eight inches in length and not less than one inch in width;
(ii) placed so that the bottom of the mark is not less than three feet from the ground or more than five feet from the ground; and
(iii) placed at locations that are readily visible to any person approaching the property and no more than:
(a) 100 feet apart on forest land; or
(b) 1,000 feet apart on land other than forest land; or
(E) the visible presence on the property of a crop grown for human consumption that is under cultivation, in the process of being harvested, or marketable if harvested at the time of entry.
If you intend to shoot someone just for being on or in your property (without intent to commit a crime, which would make it burglary), you should post NOTICE per the above guide.