I met the dog and I don't think I can handle him. How do I cancel? What should I say?

I met the dog last night and I am afraid he is too vicious for me to handle. He's a 6 year old German Shepherd and he's been re-homed 4 times! He clearly has separation and aggression issues. I don't think I have the strength to walk him and frankly I'm scared he will bite me. What can I say to the owner when I cancel? I feel guilty for not being up front at the meet and greet.