The difference can be attributed to how the word חֶ֥סֶד is understood. It is found over 200 times and is most commonly translated as mercy, kindness, or lovingkindness and most translators choose to see the most common meaning (ESV, "steadfast love").

In the Septuagint, the translator(s) considered the use in Psalm 52(51) as another instance where חֶ֥סֶד should be understood as negative:

ἀνομία, means iniquity, unrighteousness, lawlessness.

The NIV translator(s) chose to follow the LXX:

For the director of music. A maskil of David. When Doeg the Edomite had gone to Saul and told him: “David has gone to the house of Ahimelek.” Why do you boast of evil, you mighty hero? Why do you boast all day long, you who are a disgrace in the eyes of God? (Psalm 52:1 NIV)

One difference can be attributed to how the word חֶ֥סֶד is understood. It is found over 200 times and is most commonly translated as mercy, kindness, or lovingkindness (ESV, "steadfast love").

In the Septuagint, either the translator(s) misread the text (ḥāmās "violence") or considered the use in Psalm 52(51) as another instance where חֶ֥סֶד should be understood as negative:

ἀνομία, means iniquity, unrighteousness, lawlessness. Since "God" is also missing from the Greek, a misreading seems less likely. Instead, the translator understood the opening (verses 1-4) as a condemnation directed to the lawless man.

So in that sense the NIV follows the LXX: The NRSV takes a similar approach:

Why do you boast, O mighty one, of mischief done against the godly? All day long (NRSV)

However, only the NIV repeats the phrase "Why do you boast..."

For the director of music. A maskil of David. When Doeg the Edomite had gone to Saul and told him: “David has gone to the house of Ahimelek.” Why do you boast of evil, you mighty hero? Why do you boast all day long, you who are a disgrace in the eyes of God? (Psalm 52:1 NIV)

Without having an explanation or a translator note, I see two possibilities. One, the NIV sought to clarify the LXX. The statement: "Why do you boast in malice, O powerful one, of lawlessness all day long?" does imply two types of boasting, one in malice and the other of lawlessness. The other is, the translator understands the passage is describing both Doeg and David:

• Doeg boasts of evil
• David, who is a disgrace in God's eyes, boasts all day long

In this case Psalm 52 parallels Psalm 51 which has the inscription "For the director of music. A psalm of David. When the prophet Nathan came to him after David had committed adultery with Bathsheba."