Joel Hardy who runs the Mormon Haven Website, was posed the following question:
How come in the Sermon on the Mount and Sermon at the Temple (e.g. see 3 Nephi 12:43-44 and the parallel verse in Matthew) and Romans 12:14 we are told to bless our enemies and not to curse them while the Doctrine and Covenants insteads sides with the old testament's support of cursing enemies, as opposed to the New testament that rejects it? (see D&C 103:24-25 and 124:93)
Joel answered thusly:
"But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;" (Matt. 5: 44)
"Bless them which persecute you: bless, and curse not." (Romans 12:14)
"And inasmuch as mine enemies come against you to drive you from my goodly land, which I have consecrated to be the land of Zion, even from your own lands after these testimonies, which ye have brought before me against them, ye shall curse them;
And whomsoever ye curse, I will curse, and ye shall avenge me of mine enemies." (D&C 103:24-25)
God alone is the only one who can judge who should be cursed(something He did in both the Old and New Testamant, Gen. 3:17, Gen. 5:29, Gen. 12:3, Mark 11:21).
The circumstances surrounding the D&C scriptures represented not just enemies against an individual man, but an even more important threat against the restoration of God's gospel to the earth. Because of this, God Himself gave Joseph Smith the permission and power to curse those who violently fought against the Kingdom of God so that it might be preserved.
Interestingly, early Christian inscriptions provide evidence that they would “curse” opponents on occasion. The following examples (just three of many) come from Gary J. Johnson, Early-Christian Epitaphs from Anatolia (Texts and Translations 35; Early Christian Literature Series 8; Atlanta: Scholars Press, 1995).
From a marble stele from Perinthos-Herakleia (Thrace), we read (p. 55):
If anyone should harm this tomb,
he will leave behind orphaned children
and a bereaved wife.
He will run totally in fire; he will
die under the hand of evil men.
he will leave behind orphaned children
and a bereaved wife.
He will run totally in fire; he will
die under the hand of evil men.
In an inscription “to the happy (dead)” from Bomos from Ishikli/Eumeneia (p. 87), we read:
If any other endeavors
to inter someone (here),
he will reckon with the living God.
to inter someone (here),
he will reckon with the living God.
Finally, from a stele from Iznik/Nikaea (p. 151), we read:
If any other should violate it,
he will give account to God
on Judgment Day
The phrase “on Judgment day” translates εν ημερα κρις[εως] (p. 150), showing that not just a temporary curse/punishment is in view, but eschatological judgement/condemnation!