Very interesting. I had not noticed or been aware of this.
Calvin specifically pronounces on verse 15 of Romans 3:
The expression which Paul adds from Isaiah, Destruction and misery are in their ways, is a most striking one, for it is a description of ferocity of immeasurable barbarity, which produces solitude and waste by destroying everything wherever it goes...
There follows the phrase, The way of peace they have not known. They are so habituated to rapine, acts of violence and wrong, savagery and cruelty, that they do not know how to act in a kind or friendly way.
No doubt he was aware of the LXX's reading. This quote would be from Isaiah 59:7, 8.
Keil and Delitzsch, in their comments on Psalm 14:3, say:
The citations of the apostle which follow his quotation of the Psalm...were early incorporated in the [Koine] of the LXX. They appear as an integral part of it in the Cod. Alex. [and he lists a few more odd places where it is found in text or margin -SMR]...Origen rightly excluded this apostolic Mosaic work of Old Testament quotations from his text of the Psalm, and the true representation of the matter is to be found in Jerome, in the preface to the xvi. book of his commentary on Isaiah.
Lastly I submit Douglas Moo's opinion (from his NICNT commentary,
The Epistle To the Romans (Grand Rapids: Eerdmans, 1996):
The inclusion of Romans 3:13-18 in several MSS of the LXX of Psalm 14 is a striking example of the influence of Christian scribes on the transmission of the LXX. (See S-H for a thorough discussion). (p. 203, fn. 28) [S-H refers to A Critical and Exegetical Commentary on the Epistle to the Romans, by William Sanday and Arthur C. Headlam (ICC. Edinburgh: T & T Clark, 1902)]
What Moo is saying, explicitly -- as Origen implicitly, K&D concurring -- is that the LXX's reading in Psalm 14:3 came from Romans via Christian scribes, and not the other way around, i.e., from the LXX into Romans.
Thank you for highlighting this interesting oddity.
Steve