Don't read if you don't want Tsukihime spoilers.

[hider=Spoiler]
Shiki Tohno last name is actually Nanaya, and comes from a clan of Demon Hunting 'Assassins'. They were psychics who possessed 'Pure Eyes' that focuses solely on exterminating demons, and developed a unique murderous impulse against non-humans. This is why Shiki attacked Arcuied, because he unconsciously sensed she not human and was overcome with an uncontrollable desire to murder her. 
[/hider]