1. Don't say "this is really hard for me." Remember, it isn't personal, it is business. Firing someone because he or she didn't perform well isn't a hard decision to make.
2. We're going in a different direction. This sentence is not a valid reason to let someone go. In fact, the best thing you can do for the person you are firing is let them know where he or she failed so the action can be corrected in the future.
3. "We're cutting staff." Don't say this unless it is true. Lying about this can lead to a lawsuit.
There are times to say what is really on your mind. When you're firing someone- remember, this isn't one of those times. Personal reasons can lead to issues within your company so be direct and be swift.