Now let’s say my boss had a secret affair. Now I know if I said “Either give me a million dollars or I’ll tell everyone,” would be blackmail, against the law.
But what if I just said “I hate the guy and I’m gonna tell everyone.”
Is there any law or suit potential there. Like alienating affection or something like that.
Sure it could get you fired but as long as he really was having an affair, is there any legal or civil liability?
Prove it or it’s slander. If you can prove it then there really isn’t much he could sue for and win. Of course, there is always his revenge to worry about . . .
There are laws against slander and liable. If you publically said (or wrote) that your boss was having an affair, and it turns out he wasn’t, he could certainly take you to court. But unless you lie under oath I doubt you would be arrested for it. Of course you probably wouldn’t have a job…
Of course, we know you meant “libel”. 
If you are a Doctor, shrink, lawyer or clergyman, you could get in a certain amount of trouble.
Of course not, what a ridiculous idea. How could that possibly even work considering the 1st amendment.
As mentioned above, he could sue you for slandering his good name. At this point, you’d better be able to prove he’s having an affair.
How did you find out about the affair? If you did so in an intrusive manner that would be objectionable to a reasonable person, e.g., reading his email and bugging his bedroom, he could have a claim for intrusion of his private affairs and seclusion.
The First Amendment is possibly not as broad as you conceive it to be. It mostly protects you from * government * censorship. It’s extension into libel and slander cases is mostly because you need the power of the government behind you in order to sue. However, there are certain rights of others that conflict with your right of free speech. People have a right to privacy, for example. So if you have enough evidence to conclusively demonstrate, for example, that someone is having an affair, you better be prepared to explain how you legally came by that knowledge (without, say, peeping in windows).
Here’s a possibly apropos case in which a sheriff’s deputy who was accused of having public sex in a hot tub is now counter-suing for conspiracy (and, I think, defamation and invasion of privacy).