King Sombra’s rule wasn’t as bad as most ponies were lead to believe, at least, not at first. In fact, a lot of the crystal ponies were perfectly happy. Sure, he was iron-fisted, but as long as the ponies worked hard and loved their king, they led happy, fulfilling lives.
However, he was still feared for one reason; his treatment of any creature that wasn’t a crystal pony. Being a normal unicorn, he was raised to believe that crystal ponies were a superior race. When he came to realize that he wasn’t one of them himself, he came to the conclusion that he was made king to spread the message of their superiority.
Any non-crystal being that dared to enter his kingdom was immediately enslaved. Now, at the time, he wasn’t a monster, and he would give them a fair chance. After all, inferior creatures made good servants and laborers, right? So every non-crystal was made either a castle-servant or mine-worker. Those who worked hard were allowed to lead normal lives (albeit under the hooves of the Crystal ponies), those who didn’t were executed.
The Crystal Ponies could only take so many public executions before their fear of Sombra began to show, and he realized this quickly. He gradually became less of an iron-fisted king and more of a ruthless tyrant, threatening to send anypony that dared to oppose him to the same fate as all the griffins and dragons he slaughtered. Eventually, this warped him into the merciless, tyrannical monster he’s known as today.