Hell no it's not a battery killer...it's a battery saver if anything. This is easly one of my personal Top 5 apps.
At present, you only have options to 'never' turn off the screen or turn it off after 'time' and neither option is really suffecient. There is also the menu option of keeping the screen 'on' if the phone is plugged in and while it does keep the screen on, it keeps it in a dimmed state. If I try and use 'never', I inevitably forget to manually turn the screen off, thus leading to a huge waste in battery life. If I try and use 'timed', then a) the timed length is generally too long if I'm just checking a text someone sent me (leading to lost battery life...but hey, the screen will turn back off) or b) it's too short for reading articles where you don't input a keypress that automatically renews the screen timer from turning the screen off.
Screebl Lite is, IMO, just one of those 'must have' apps as it puts functionality into the phone that is transparent to the user. When it comes down to it, I don't worry about my phones display shutting off when I don't want it to, nor worry about it staying on forever and killing the battery.