Greentext

When the bible said it's okay to beat your slaves so severely that they die, as long as you they survive a few days before they die, it was wrong for me to characterise that as murder, I was hasty. But the bible clearly says that slavery is legitimate when the enslaved weren't treated the same as the free by the law. The fact that they're both made in God's image was clearly not relevant.

The Bible isn't a legal document, this was regarding a law for an ancient nation that doesn't exist anymore that was built on compromises with people who have hard hearts.

No I wouldn't, I would consider their argument. I'm not dogmatically committed to believing Christianity is bad. Are you dogmatically committed to believing Christianity is good?

Depends on what our definition of Christianity is - I wouldn't define enslaving people as a Christian practice.

Christianity saw slavery for 1500 years and said "meh"

You have a misconception here that Christianity was popular for the first 1500 years. For the first few centuries of its existence, Christians were persecuted quite severely. Then for the next millenia, it was a religion for rulers generally, and the general populace often held on to pagan beliefs.

In the middle ages, slavery did naturally fizzle out in western Europe, the debate was generally started again whenever the whole translatlantic slave trade started in the 1500s. In 1772, a British court ruled that it was impossible for a man to be property of another man Lord Mansfield (note: dated language):

The state of slavery is of such a nature that it is incapable of being introduced on any reasons, moral or political, but only by positive law, which preserves its force long after the reasons, occasions, and time itself from whence it was created, is erased from memory. It is so odious, that nothing can be suffered to support it, but positive law. Whatever inconveniences, therefore, may follow from the decision, I cannot say this case is allowed or approved by the law of England; and therefore the black must be discharged.

so at least by this point it was already seen as not legal in mainland UK. Eventually the UK banned it in the whole empire in 1833.

Revelation 18:10-17

They will stand far off, in fear of her torment, and say, “Alas! Alas! You great city, you mighty city, Babylon! For in a single hour your judgment has come.” And the merchants of the earth weep and mourn for her, since no one buys their cargo anymore, cargo of gold, silver, jewels, pearls, fine linen, purple cloth, silk, scarlet cloth, all kinds of scented wood, all kinds of articles of ivory, all kinds of articles of costly wood, bronze, iron and marble, cinnamon, spice, incense, myrrh, frankincense, wine, oil, fine flour, wheat, cattle and sheep, horses and chariots, and slaves, that is, human souls. “The fruit for which your soul longed has gone from you, and all your delicacies and your splendors are lost to you, never to be found again!” The merchants of these wares, who gained wealth from her, will stand far off, in fear of her torment, weeping and mourning aloud, “Alas, alas, for the great city that was clothed in fine linen, in purple and scarlet, adorned with gold, with jewels, and with pearls! For in a single hour all this wealth has been laid waste.” And all shipmasters and seafaring men, sailors and all whose trade is on the sea, stood far off

The abolitionist movement was primarily driven by Christians. A notable figure was William Wilberforce who become one after converting to Evangelical Christianity. The hymn "Amazing Grace" was written by a former slave-trader turned abolitionist, who upon conversion to Christianity, penned the lyrics ".. That saved a wretch like me; I once was lost, but now I'm found; was blind, but now I see!" out of repentance. William Wilberforce eventually proposed the ban on the transatlantic slave trade which was equated to piracy in 1807.

In the USA, slavery was essentially ended by the Great Awakening which was a time marked by mass conversion to Christianity among the peasantry.

Christianity was definitely a major, if not an essential factor, towards ending the slave trade.