So this riddle is solved by making a paradox statement, a fancy word for something that isn't true or untrue.
The statement must include the wizard killing you. This means that once you say this the knight can not kill you as it would make your statement false and he only kills people who tell the truth for some undisclosed reason. The paradox is the wizard bit. The wizards kills lairs. If you state he will kill you than the act of him killing you makes you be truthful and he doesn't kill truthful people. So you haven't told the truth or a lie until he reacts so neither can do anything.
@nerdig is the winner here, so well done.
Rewards have now been paid.