A combination of a LOT of factors, ranging from the way he was raised (his mother's abuse + the encouragement of truly awful behavior he got as baby English nobility) to untreated inherited mental issues and rampant sadism to the moral gutting he experienced first at the hands of his twin brother, then scraping by a living on the streets of Bali, then as right hand to a crime lord. But ultimately Alec has no desire to be a good person. Any of these things could have been resisted if he wanted to; he doesn't. Why exactly he doesn't is up for debate.