NEW YORK — A homeless suspect accused of following a woman into her Manhattan building then stabbing her to death inside her apartment was charged Monday with murder, police said. It’s still not clear why Assamad Nash allegedly killed 35-year-old Christina Yuna Lee, but police sources said detectives do not believe she was targeted because she was Asian. The victim, despite fighting back against Nash, 25, was killed early Sunday. Her cries for help prompted frantic 911 calls from neighbors about an attack unfolding her apartment on the Lower East Side. When police got there the blood-soaked su…