Some folks are enamored of the idea of “shooting dead an intruder in one’s house without asking questions,” but I’m not one of them.
Necessary self-defense has always been a basic part of common law, but it is quite another thing to kill a man or woman on the sole pretext that that person is an intruder, whether or not the person constitutes a genuine threat to anybody in particular. Even jurisdictions that have stand-your-ground laws, or “castle” laws, generally require that you have a genuine fear of harm, or at least a genuine fear of a serious felony, before you are justified in using deadly force. And even then, the level of force must be necessary. Stephen Maitzen is entirely right when he points out that an undocumented alien is almost certainly not a genuine and immediate threat to you. Beyond this point, however, the notion that you can lawfully kill someone just because that person has intruded into your home now circulates broadly, and it is, in fact, legally false. You have no such legal right. The notion is also morally pernicious, because it has encouraged a number of gun owners to commit murder—for which they have rightly been prosecuted.
(I say this, by the way, as someone who enjoys shooting guns—indeed, I never saw a gun I didn’t want to shoot. And I admit that there can be circumstances where there is simply no opportunity to talk and where you must fire to protect yourself or others. But guns should be kept out of the hands of fools. Many developing nations have higher homicide rates than the U.S. does, but among developed countries, the U.S. per-capita homicide rate stands alone, and so does the ease with which Americans obtain guns.)
John Locke discusses the use of deadly force as a means of necessary self-defense in his Second Treatise of Government, and he has at least two memorable lines that seem relevant here: “When all cannot be preserved, the safety of the innocent is to be preferred.” And yet, “Even the guilty are to be spared, where it can prove no prejudice to the innocent.” (These lines appear in paragraphs 16 and 159.)