Not under all circumstances it isn't. If the trespasser is trying to break into your house, use of deadly force is lawful, at least in Texas. But I guess that then removes the "simple trespassing" criteria
§ 9.32. DEADLY FORCE IN DEFENSE OF PERSON.
(a) A person is justified in using deadly force against another:
(1) if he would be justified in using force against the other under Section 9.31;
(2) if a reasonable person in the actor's situation would not have retreated; and
(3) when and to the degree he reasonably believes the deadly force is immediately necessary:
(A) to protect himself against the other's use or attempted use of unlawful deadly force; or
(B) to prevent the other's imminent commission of aggravated kidnapping, murder, sexual assault, aggravated sexual assault, robbery, or aggravated robbery.
(b) The requirement imposed by Subsection (a)(2) does not apply to an actor who uses force against a person who is at the time of the use of force committing an offense of unlawful entry in the habitation of the actor.
From Chapter 9, Texas Penal Code.
I don't know what the laws are in Mississippi, but I wouldn't expect the laws in Mississippi to be signifigantly less enlightened in this regard. Certainly the laws on use of deadly force vary greatly from state to state, with those in the South being generally more protective of individual defense, and less protective of aggressors.
You're correct about why I limited my statement to "simple trespass". But does Texas allow for no-knock warrants? If it does how do you provide for the young officer that's made a mistake or is acting on bad information? The Maye situation teaches me that even if my door is being broken down and men are coming in, I have to pause long enough to establish that it's not the police and/or loudly ask who's breaking down my door. However, I must admit that prior to shooting to kill, I can not imagine not yelling a warning and firing off a couple of warning shots.