There may be times when RoboKiller will show that it is using most or all of your battery life. There is no need to be confused by this. If you only use a few apps during the day, your device is going to show that those apps are using all of your battery life since they have the most screen time.
One way you can reduce this is by making sure you force close the RoboKiller app so it is not running in the background. The app will still block calls and log them into your account for you to view when you reopen the app. You can only reduce an app's "on screen" battery usage by reducing the amount of time you use the app. In other words, don't open it and it won't be "on screen".