Electronic Dog Training Collars are used for problem solving (such as jumping on people, chasing cars, stealing food, avoiding snakes) or for more general training (such as obedience training or hunting dog training). Aggressive behavior is NOT a suitable candidate for electronic collars under any circumstances without a behavior specialist supervising the activity.
Many people think of a "shock collar" as a last resort, designed to punish a dog when he performs poorly. However, used properly, a remote training collar can be a boon in ANY training situation. It is used to communicate, not punish. Most collars have a warning tone before the stimulation, and most have very mild stim to remind or reinforce.
Training should be accomplished at the lowest stimulation level that gets an "attention" response without causing any fear or yipping pain. Your dog should look up at you when he feels the stimulation. His neck muscle might give a small twitch, but no vocalization, lowered ear set, cowering or otherwise showing fear should be apparent. Used properly, he will learn to "listen" for the "feel" and follow your diretons in any circumstance. It can give a dog confidence and give you peace of mind.