Disclaimer: Angel and all characters in said show belong to Joss Whedon
Illyria isn't Fred. Spike knows this. She doesn't smell, move, or even look all that much like her.
He tells himself this often, and tries not to feel guilty about punching her in the face. Sometimes it works. The fact that Illyria can and does kick him into walls helps with the not feeling bad about it.
Fred loved tacos, shared crackers with him, and laughed when he offered to let her try his otter blood. Illyria doesn't eat anything, just strides down the halls, craving who-knows-what.
Also, Fred was always nice to him, and Illyria's kind of a bitch.