I do not know about the MS law, but in PA whether on your property or in public, if a person presents a visible deadly weapon and acts in a threatening manner toward you using deadly force is justifiable. That also applies to protecting others who are so threatened. I have my own simple rule: I would rather go on trial for wounding or killing someone than have them go on trial for wounding or killing me. If a person with a knife treated me anywhere he or she would be quickly looking down the barrel of a 9mm Walther,
Looking at PA statutes which are typical of an old state compiling old court decisions into code. They are rather complex with a bunch of caveats, restating basic principles of self defense in multiple parts, etc. I would advise going through something like Branca's Law of Self Defense for your state.
The major issue is that the PA law has an unstated reasonable person objective std. which is not found directly in the statute stated as such. A reasonable actor would be defined by the jury looking at the facts in the case, the judge--both facts and law, the statute's wording, and caselaw interpreting that wording. PA affords less protection if you are confronting a trespasser, even on your own land. See section two below. In other words, going off to protect your land from a trespasser might be considered unreasonable if you take a gun or knife with you to the confrontation. From my reading, absent doing any caselaw analysis, is that aside from your home, workplace where the castle doctrine mentioned below applies, that confronting others on your property with the chance of escalation is not a wise idea. Best in this case to call the cops or retreat from the situation if you stumble across them on your property. The person on your property could be an undercover/off duty cop looking for a fleeing suspect, a game warden, a lost hunter, a surveyor, an assessor, or a meth crackhead. Any by the last would risk a deadly confrontation that could go south real quick.
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https://codes.findlaw.com/pa/title-18-pacsa-crimes-and-offenses/pa-csa-sect-18-507.html"
Here is the section referring to deadly force and property,
"(2)
The use of force to prevent or terminate a trespass is not justifiable under this section if the actor knows that the exclusion of the trespasser will expose him to substantial danger of serious bodily injury.
(3) The use of force to prevent an entry or reentry upon land or the recaption of movable property is not justifiable under this section, although the actor believes that such reentry or caption is unlawful, if:
(i) the reentry or recaption is made by or on behalf of a person who was actually dispossessed of the property; and
(ii) it is otherwise justifiable under subsection (a)(2).
(4) (i) The use of deadly force is justifiable under this section if:
(A) there has been an entry into the actor's dwelling;
(B) the actor neither believes nor has reason to believe that the entry is lawful; and
(C) the actor neither believes nor has reason to believe that force less than deadly force would be adequate to terminate the entry. (editorial comment, bolded text in part 4--this is the Castle Doctrine dealing only with the home and not out buildings nor land)
(ii) If the conditions of justification provided in subparagraph (i) have not been met, the use of deadly force is not justifiable under this section unless the actor believes that:
(A) the person against whom the force is used is attempting to dispossess him of his dwelling otherwise than under a claim of right to its possession; or
(B) such force is necessary to prevent the commission of a felony in the dwelling.
Here is the general self defense statute of PA,
https://codes.findlaw.com/pa/title-18-pacsa-crimes-and-offenses/pa-csa-sect-18-505.html for the public.
And defense of others statute of PA,
https://codes.findlaw.com/pa/title-18-pacsa-crimes-and-offenses/pa-csa-sect-18-506.html
The critical issue on use of lethal force is whether or not the person doing the action (you if acting in self defense) has a reasonable (that is for the jury, judge, statute definitions, and past court cases to decide) belief that the other person intends to "(2) The use of deadly force is not justifiable under this section unless
the actor believes that such force is necessary to protect himself against death, serious bodily injury, kidnapping or sexual intercourse compelled by force or threat" Pennsylvania Statutes Title 18 Pa.C.S.A. Crimes and Offenses § 505.
The unstated presumption when they refer to actor (you) is that you are acting an objectively reasonable manner defined by the jury, judge, statutes, and caselaw. These unstated presumptions are why it is difficult to read specific code and apply it to your situation.
Here is an PA attorney's interpretation of PA law regarding self defense but leaving out the substantial caveats in the statutes (and maybe in the caselaw).
https://statelaws.findlaw.com/pennsylvania-law/pennsylvania-self-defense-laws.html
So apply this to the original post, someone is on someone else's land. Let say the guy was a hunter that got lost or even a poacher and they have both rifle and knife. You are still not allowed to shoot them unless they make overt threats to you indicating a risk of serious injury or death. Example, they are raising the rifle to firing position on the shoulder to point at you and saying something like "I gonna teach you a lesson." Then most reasonable people would believe that was a serious threat to life and limb of the target. But if the rifle is slung, the knife is in a sheath, a jury would take a very dim view of arguing that a reasonable threat to life or serious injury existed.
