As the history of Middle Earth down through the ages demonstrates, even the elves were not entirely immune to the corruption of evil (Melkor/Sauron/etc.) Not that any, as far as I can remember, ever reached the point of being directly under their service...but certainly there were some who fell under their sway and made some choices that had catastrophic results for elvendom in Middle Earth. (The two kinslayings would be the best examples, but one could probably think of others as well.)
So when an elf (even a Lorien elf, who presumably has been gone from Lorien for a LONG time) suddenly shows up, having consorted with dwarves and who-knows-what-other influences, in a time where darkness is pressing in on all sides, I could see the Lorien guardians being a little on the suspicious side. Shooting a fellow elf dead is perhaps extreme, but within the realm of believability. Remember in the books, when Legolas begins climbing the tree and the Galadhrim guards above order him to stop, he freezes and in a very emphatic way tells the others not to so much as MOVE. I think in that moment Legolas certainly had the impression that deadly force was a very real option.
Officer and Co-Founding Member, Ring of Destiny
Gladden server Á auta mornië! i cala tula lennar! ("Flee, darkness! The light comes upon you!")
And for further reference , you had the episodes of the kinslayings in the first age ,Eol,Maeglin and the sons of Feanor didn't really behave in an exemplary manner, and also reports of elves released by Morgoth or escaped that wern't trusted and sometimes completely shunned by their own kin in Doriath and Nargothrond,Thingol banning Noldorin speech after hearing of the Kinslayings,so yes, there is previous as far as Elves not getting on and coming to grief sometimes amongst themselves.