For colic I've always thought of it as most animals, when their stomach disagrees with something, has something it can not handle it vomits it out. Horses on the other hand do not have that capability and what ever is bothering their stomach stays there.
As far as I know the main reason for a cause of death from Colic is when the horse constantly rolls to escape the pain, then they twist their intestines, and after that they have to be euthanized. Or from the original problem.
Just like people are more or less susceptible to colds because of their different immune systems, horses are more or less prone to getting colic or founder. All three horses I have owned have never colicked, I have done free pasture with feedings, stalled with only hay feedings. Fed only alfalfa and grain, had horses that had a lot of exercise in various amounts of weather and horses that have been on stall rest for months. None of them colicked because they are not as prone to it.
While on the other hand I had a neighbor that bought a horse knowing it had colicked a few times in the past. The horse was fine until the owner stopped riding and fed too much hay, and we had a heat streak. The horse became obese, colicked and by the time the owner found it the horse had been down for hours and had twisted an intestine. The horse was sadly euthanized. But the process that took to getting to that point took well over 7 months, and all extreme possibilities.
I know very little about founder other than it is from over feeding rich food and causes extreme lamness. But I have never seen a horse that had foundered. Its less common than colic.