Being a jerk means being an unlikable person especially one who is cruel, rude, or small-minded.
Being an asshole means a person who is intentionally cruel, obnoxious, and heartless. Assholes are most often male. A female who follows these traits is referred to as a bitch. Assholes can take any form. Knowing this allows you to spot assholes at any time or place.
As a DOM calls for you to be dominant, be a leader, be the alpha, but don't be an asshole. Treat people well. It's nicer to express your intentions directly than it is to mask them under the disguise of pleasing everybody. People love giving you what you want if you can make them feel great about themselves. Being an asshole just to be "alpha" is the same as being the "nice guy"; you're still just putting up a fake front to get what you want rather than being direct.
Doms are not nice guys either. You can avoid being the "nice guy" without being a dick. Dominant leaders are still nice to people, they just also get what they want. The "nice guy" is a problem because he is masking his intentions under FAKE kindness to get what he wants. He is also a coward. The very important difference between the "nice guy" and a dominant, an alpha male is not that the dominant guy is mean to people, it's just that he isn't ashamed of his true intentions and he goes after what he wants without faking it. People like helping a guy who is both courteous and always knows what he wants. People don't like helping an asshole just because the asshole is being direct.