Moved to HTC One section.
You might be able to find battery percentage In Settings > Display on your device. I know Samsung devices have this you might too, but if not you could root like taylor said.
Don't know about your first question as I don't use that app.
What nickdalzell said pretty much sums it up. Don't ever use app killers though as it can make your battery life worse and just isn't good for the device.