Yes, it can kill quickly. There are different strains, some are more virulent than others, and individual immune systems vary. It's always best to start treatment as soon as possible when it's suspected so that damage to the digestive tract can be minimized. It's unfortunately always a risk...
What do her droppings look like? With lethargy it can be many things but at this age coccidiosis is always a possibility. Corid is the treatment and is safe even if it's something else, so if that is suspected I would go ahead and treat.