Is it illegal to take at a swing at someone?

[your best enemy =)]

If someone threatens to kill you, but doesn't do anything about it, would be illegal to punch them in self defense? Would that be considered self defense?

 

[leo lion]

no you are sadly mistaken

 

[Lexie]

Only if you have a reasonable belief he is about to act on that threat. Depends on the circumstances. Generally, words alone are not enough.

 

[Starship Pain]

Thats really a IFFY issue, it depends on the laws in the city/state you are in. Generally speaking if you can prove that you felt extremely physically threatened by the person and you can prove that they have verbally threatened to kill you in the past you probably could get away with it.
Push comes to shove, in a situation like that if there are no witnesses around it is your word against theirs. Best thing to do is tell the cops he HIT you first even if he didnt.