Remember it is illegal to log into someone elses account and an invation of their privacy which would land you in a lot of trouble. Nasty people just are not worth getting into trouble for.
If they are being abusive or threatening you can report their offensive posts, block them or contact facebook which could possibly help. Hope it gets better for you.
when you say "got their email" do you mean you know their email address, or you have access to their email account?
if all you have is that you know their email address, then basically, NO, you wont get anywhere.. There are probably some nasty/blackhat/immoral/illegal ways to go about it but you probably dont want those risks and we dont talk about them here anyway.
For problems logging in email HELP@GrownUpGeek.com