So it will work?
I don't mind if the guards kill it as long as I can loot the horn from it after. Will the horn show up if the daedric quest for it isn't active?
Yes, it will work. Remember that the unicorn is always level 15 and does not level up. Therefore, this will work best at lower levels.
Reference: http://www.uesp.net/wiki/Oblivion:Unicorn
Yes, the horn will show up if the quest is not active.
The unicorn horn is a quest item. Quote UESP wiki: '
You do not have to be at level 17 for the unicorn to appear in the Grove. You can ride or kill the unicorn at any point in the game. However, killing the unicorn is no easier at low levels (the unicorn's statistics are not leveled). Also, once you have obtained the Unicorn Horn it is a quest item which, while weightless, still cannot be removed from your inventory until you complete this quest.'
Reference: http://www.uesp.net/wiki/Oblivion:Hircine#Notes
The unicorn horn cannot be used to complete the Blood of the Daedra portion of the Main Quest. Only the 15 items listed at this link can be used for that:
http://www.uesp.net/wiki/Oblivion:Blood_of_the_Daedra#Blessings_of_the_Fifteen
In the notes at the bottom of the list in UESP wiki, you will find this statement:
'
Only the fifteen items listed above can be used for this quest. All other items, even ones with Daedric origins (e.g., Mehrunes Razor, the Staff of the Everscamp, Staff of Sheogorath, etc.), cannot be used.'