When he is disrespectful, he will justify it by either blaming his past or her. She feels sexual chemistry, but she clearly doesn't trust that she will ever get her other needs satisfied. By not understanding the signs of unhealthy chemistry, we can easily get involved with someone who is clearly not right for us. No Say something positive before or after you say no. Make it a genuine compliment.
Bad news is always easy to take if there is some good news along with it.
State your position simply. Avoid getting into the realm of honest critical feedback, especially if you don't plan on seeing this person again. You are not responsible for giving a reason for saying no. You can if you want, but you don't have to. Your no should be clear and effective.