As I understand it, most cruelty was due to the fact that they were trying to limit revolts as much as possible, and the only way they really thought to do this was through the usage of deterrents. In the event of a revolt, to say consequences would be severe would be a massive understatement.

To give you a few examples: One fifteen year old boy had bricks strapped to his feet/neck and thrown overboard as recompense for an attempted insurrection. A woman was subsequently tied up and repeatedly slashed at the stomach until she died, whilst a man was to chopped into pieces only to have his severed parts passed around to the remaining slaves on the ship whilst he still lived. In a different instance, an African cook aboard a ship was forced to boil water in a cauldron and pour it over rebelling slaves.

Approximately 10-20% of slave ships experienced some form of rebellion, and this is with all the preventative measures in place on ships including the separation of slaves of similar language (they would chain slaves together to people with entirely different languages to prevent communication), the building of 'barricados' (essentially walls) in the middle of the ship to separate men and women, and the placing on netting on the side of the ships to prevent escaping overboard. They also placed turrets above deck that would be manned to give an eagle eye view of all slaves during the day, meaning they were constantly watched. Furthermore, life as a slave on a ship meant having a heavily regimented daily routine, which essentially meant waking up at 8am, spending time on deck chained to complete strangers until 6pm, and then sleeping. In Oloudah Equiano's account of his journey on the Middle Passage, he claims he would have jumped overboard if it weren't for netting stopping him. He further claimed that if he had 12 worlds he would exchange them all just to switch his position with the worst treated slave on the African continent (they had their own slave trades).

Everything essentially came down to money and efficiency. Whilst sailors were in no way subjected to the same conditions of slaves, they did not exactly have an easy ride due to the lack of supplies such as food available on ships.This would mean that merchants in England would try to buy as little food as possible for the journeys so they could increase their profit margins. Remember, the more slaves you can fit on a ship, the more money you stand to earn once you get to the Americas, so the idea was essentially to cram in as many slaves as possible. This led to horrifying conditions below deck that led to slaves having to 'spoon' each other on either side, as they each only had 3 ft of space. Yes 3 ft. As Robert Norris put it, they "had no so much room as a man in his coffin". It is said that some became so agitated in the heat (Having up to 600 people below deck meant over 40°c temperatures) that the men would strangle each other in frustration just to get more room, whilst the women would dig their nails into other's skulls. Many more suffocated.

The loss of a slave was not ideal for their owners, so believe it or not they did actually try to keep them alive (just about). It is for this exact reason that Equiano was flogged for refusing to eat upon his arrival on the ship. If you were found to be sick, they would put you in a slightly more spacious and comfortable area in the middle of the deck (also so you didn't infect other slaves). They were heavily inspected upon their arrivals both on and off ship, with surgeons going as far as to lick their sweat to determine if they had any diseases. They did not tend to kill them for the fun of it, so if they did, they did it for a reason such as punishment.

What I've talked about here is directed more towards the Middle Passage, although I will say that in English colonies, slaves were generally treated better than under their Spanish counterparts so that they could benefit from creole (people birthed from) slaves. This is partially why there is more of an African diaspora in the US today as opposed to South America.

[Edit- Clarity/grammar]