He didn't 'have slavery abolished', but he was a leading campaigner for it to abolished.
His campaigning resulted in the abolition of slaves being bought or sold throughout the British colonies. It didn't free those already enslaved.
He had a Damascene conversion and an almost total reversal of his previous beliefs. Although he continued to support social reforms internationally, his support for domestic political and regressive legislation continued.
He only became a leading campaigner for the abolition of slavery after being in contact with other abolitionists who persuaded him to take a leading role.
What!!
11 million of them transported across thousands of miles of ocean in such shocking conditions that 1.5 million never survived?
Wishful thinking on your part?