Verified answer

1) No. Many people behave unethically through following their desires. For Kantians in particular, following desires and preferences is basing action on the phenomenal world, and cannot in itself give rise to ethical action. And many religious systems of ethics say that desires and preferences are compromised by sin, ignorance, etc.

2) Yes, I believe they can. Conscience is just rational thought applied to past actions, and so is very fallible. Of course, some Christians might say that it is not a true conscience if it leads to evil, but then they are faced with the problem of saying what true conscience is and how we recognise it.

1: no

2: yes

sorry, I'm too tired to expand on the topic right now