*Sigh* (My thoughts on Sephiroth-CORRECT SPELLING OF SEPHIROTH!)

He killed her from behind for a reason so Cloud could see him do it, see it coming but be able to do nothing to stop him. By going from behind Cloud knew what he should have done but didn't do anything. His game plan was to mess with Cloud's head.

And also his mutant mother remark...
How many Villians cared about a girl? He wanted Jenova for power. He controls Jenova (which explains why you fight Jenova because Sephiroth needed someone to fight you so he sicks mother on you).