Hi Longtimer and Odola,
Turin's purported actions come from a prophecy by Mandos in 'The Lost Road and other Writings'.
Now, do we consider this source to be: a) authoritative; and b) something that would be known to Elrond? (It certainly would not be known to a first-time reader of TLOTR, and is not an interpretation which could be gleaned from a close reading of that work.)
a) I do not consider any of the Legendarium published by Christopher Tolkien to be authoritative. JRRT kept revising all his stuff, and we do not know if he might have revised (or eliminated) this prophecy before he ever came up with something he thought ready to publish.
b) But, if you want to use Legendarium material to try to interpret TLOTR (a dubious undertaking), then, would Elrond know about this prophecy? Well, if one considers all writings in the Legendarium 'in frame', then, none of 'The History of Middle-earth is strictly 'in frame'. It is all an exploration of the evolution of JRRT's writings, rather than excerpts from 'The Red Book of Westmarch', as translated from Elvish by Bilbo, and from Westron by JRRT. If we still want to use this out of frame material to interpret TLOTR, as this prophecy does not appear in 'The Silmarillion', then 'in frame' it is not part of the Elvish Lore from Rivendell which Bilbo translated. This might make it less likely (though not impossible) that Elrond knows of it. However, the transmission mechanism by which a prophecy of Mandos could be known by Elrond, is not at all clear.
So, one can use this Legendarium material to speculate that it is Turin's role in the Dagor Dagorath, of 'dealing unto Morgoth his death and final end', with his black sword, which prompts Elrond to include him in the list of mighty Elf-friends. But, use of Legendarium material is dubious (there are plenty of contradictions across 'The History of Middle-earth, and between that and 'The Silmarillion'). And, even if one still wanted to use it, one would have to speculate about how Elrond might have know this prophecy. It is certainly not an interpretation which could be derived from a close reading of TLOTR.