Fear of death ties in a lot with fear of pain. Any work that you do with pain is going to help you with death. A lot of people say they are not afraid of what might happen to them after death, but if you ask them would it be okay to be tortured for five hours, burnt alive, beat up some more, dragged behind a car before you die, for that they might be afraid. And that is real, that is a real story that actually happened to a young girl. Tortured for five hours, burnt but she didn't die, beat up, then dragged behind a car, before she died. So fear of pain is for most people more important than the fear of death.
Now for people who have done bad Kamma, the minute they start to think that maybe the concept of heaven and hell might be true, this can scare them a lot. If you have done a lot of bad Kamma in your life and if you consider that something will happen after this life, and that bad results will come to someone who has done bad, as most religions basically teach, then what you have to do is to start doing a lot of goodness. So you don't die with lots of regret about what you have not done. If you have done lots and lots of goodness then according to Buddhist teachings, you will get a good rebirth. So that fear of dying which is often very real for many people who have done bad things in their life, can be overcome by doing lots and lots of good things.
An analogy that might help to make this example about good and bad Kamma clearer concerns mixing salt and water. Imagine a glass that is full with water. Then take three big table spoons of salt and put them in the water, stir them in and the salt dissolves. Do you want to drink this salty water? Three table spoons of salt in a glass of water, no, you don't want to drink it, it's far too salty. The water in the glass is like our good Kamma and the salt is like our bad Kamma. When there is too much salt in this good Kamma, it is not good to drink. Now, imagine a big bathtub full of drinking water and then imagine that you put those same three tablespoons of salt in the water and then stir them in, can you drink water? Yes. And you probably wouldn't even taste the salt. So the more good Kamma you put in, the more good Kamma you make, the less the results of your bad Kamma you have made will hurt you. Keep putting in clean water and make your bathtub become a lake.