The ‘headings’ we found in a lot of Bible translations are not inspired by God, obviously. They represent only a practical device to subgroup the text in logical paragraphs. They are very useful also to search and found a given passage we have in mind.
As regards 'murder' as a verb the following site (https://www.etymonline.com/search?q=murder&ref=searchbar_searchhint; bold is mine) presents the following etymology: "c. 1200 mortheren, "to kill, slay; kill criminally, kill with premeditated malice," from Old English myrðrian, from Proto-Germanic *murthjan (source also of Old High German murdran, German mördren, Gothic maurþjan, from Proto-Germanic *murthra-."
As regards 'murder' as a noun the same site has the following definition (bold is mine): "unlawful killing of another human being by a person of sound mind with premeditated malice," c. 1300, murdre, earlier morþer, from Old English morðor (plural morþras) "secret killing of a person, unlawful killing," also "mortal sin, crime; punishment, torment, misery," from Proto-Germanic *murthran (source also of Goth maurþr, and, from a variant form of the same root, Old Saxon morth, Old Frisian morth, Old Norse morð, Middle Dutch moort, Dutch moord, German Mord "murder"), from suffixed form of PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death)."
So, it seems to me that in English language all Bible translators have to take into consideration the difference between 'to murder' and 'to kill' (or, to 'slain'). However, a number of Bible translations correctly understand this concept.
For an example, the following data drawn by NKJV Study Bible could be useful (bold is mine) in this discussion.
It has (on the passage of 2 Chr 23:12-21) the heading ‘Death of Athaliah’. The footnote on verse 10 starts with the sentence: “Most of the royal heirs Athaliah murdered...”. In the box (on page 639) titled ‘Queen Mothers’, speaking of her, we read again: “She [Athaliah] then tried to kill all the righteous heirs...” And, as final sentence this box says: “... Athaliah was overthrown and killed...”. Moreover, in the main text (the proper translation), we found the use of the verbs ‘to slay’ and ‘to kill’ (both in 23:14).
So, in this case, it seems to me the editor of NKJV Study Bible have well understand the difference between these death-related verbs, using the verb ‘to murder’ for the killings of the righteous heirs, from Athaliah’s part; as well as the use of the verb ‘to kill’ for Athaliah’s death.
I don’t know the reason why the editorial board of some Bible translations decide to use the verb ‘to murder’ in connection with the righteous death of Athaliah, though I admit this option is an ill-chosen term.
Interestingly, commenting the verse 14 (where the priests said “Do not kill her [Athaliah] in the house of the LORD”) the linked footnote of NKJV Study Bible states: “The temple was regarded as a place of sanctuary from violence”. Now, if this latter point was the basis of the priest’s scruple, this was a scruple with no divine basis (but only a personal one, or, it was based on the epoch’s ‘politically correct’ behaviour). In fact, Lord Yahweh thought differently.
In Exo 21:14, we read about God’s viewpoint on this matter: “If you plan in advance to murder someone [like Athaliah did], there’s no escape, not even by holding on to my altar (מזבח). You will be dragged off and killed.” (aptly, please compare this passage with Joab’s execution, described in 1 Kin 2:28-34)
So - we may ask - are now some translators influenced (like those priests of ancient times) by the ‘politically correct’ trend of today, that all kind of killing is divinely wrong? I hope not. Differently, they have to dig deeper into the Bible text.