1. If I say or write something bad about a person, but I have no intention of defaming him, would I still be liable for defamation?

You may still be liable for defamation.

It is not important if the one who speaks or publishes the defamatory words does not have the intention to defame the other, what is important and more relevant is whether or not reasonable persons in society think or opine that the words published have defamed the other.

The law of defamation is concerned with the effect of words on ordinary people. The standard to be applied is that of right-thinking/reasonable members of society. Under this standard, if the matter published defamed a particular person, you may be liable for that defamation.