Hypothetically, I am a soldier and I kill 5 people with a hand grenade. How might this karma be “paid” back and how might it be “healed” if I learn not to kill halfway through the numerical payback?

You are thinking of this very much as a mathematical equation akin to "an eye for an eye and a tooth for a tooth," where the destruction of five lives might mean you must die five times in a ghastly way to atone or to rebalance the karmic consequences. It is not so simple because there is much more involved than the loss of the primary lives. In addition, there are effects that are secondary and tertiary and beyond, in a series of ripples outward, the loss carrying forward to impinge on many, many, others in most cases. A high percentage of soldiers have family who love them and who will be quite devastated with their tragic loss. And this is true of their comrades in arms as well, who will miss them and whose fear will be greatly increased in losing someone they know and may be close to. All of these consequences, including the guilt felt by commanding officers and others involved in perhaps a series of events causing them to be in harm's way, may insert their own emotions into the scene and come away feeling responsibility and therefore suffer. All of this goes into the energy proceeding from the event in question and must be a factor in the rebalancing. This is unknowable to human, but the Law of Karma will keep track and will apportion out the requirements for restitution. There are always options available for healing such obligations, for in most cases what is most needed is a return to an inner balance within, to serve love and light once again and get back on track living as a spiritual being and not causing harm to others. This is the first step in healing what has come before, and healing will be proceeding all along the way. Once a person returns to a better way of living, they can repay the debts from such activity through a life or perhaps a series of lives, simply not choosing to indulge in violence. This may be enough to re-balance things. There may be more demanded, and certainly when the karmic event itself is re-awakened, this will set in motion the focusing of karmic energies from many sources back to the perpetrator, and the sentence will become due. It is sort of akin to the process of legal justice where there may be a trial and sentence, but then a period of time during which the convicted person may still be free to see to their affairs with a sentence date to commence in the future, and then must report to jail on that date to begin carrying out their sentence. So the same happens with karma. It is not an immediate debt that impinges on a person to alter their life. It may be awakened in a subsequent life or multiple lifetimes later. So what needs to happen to satisfy the karma is a higher awareness of responsibility and the wrongdoing involved in what took place. And many, many, times when a person has the suffering come back to them and feels the pain and intensity, if they bring in a request for healing they may be absolved of their further obligation. For the whole point of karma is to bring things into balance for the subsequent future. So once the payment has been made and the restitution is complete, nothing further is required. But many times if a person learns from their pain what suffering is like, and this creates a state within of vowing to never go through this again, nor cause it to others, this may satisfy the debt in and of itself, and the person may be let off the hook, so to speak. There is not a requirement to suffer indefinitely. There are ways to speed things up, and this is by getting back into alignment in a full and complete way. For if you think about this, the need is for all to be in alignment and to be in balance. So if someone comes into full alignment and inner balance, they in essence have recovered from their prior state of discord and at that point there is no further value to the universe in causing torment.