One of the hardest things to do as a manager is decide to fire someone. Particularly if you're a new manager, you tend to spend a lot of time blaming other things -- blaming yourself, blaming the goals, the business, other people. One of the things you realize after you've managed a lot of people is that if you get that feeling in your gut, then something is wrong. It doesn't automatically mean the person needs to be fired, but it does mean that you need to do the work — do the work to figure out what's wrong and whether it can be fixed.