Depending on how you define "being an asshole", yes, to some degree you have to, because you have to stand your ground sometimes to reach certain objectives, and that will be seen by some as being an asshole.
However, you can be civil about it, and keep it to a strict minimum, and still be successful; it is however far easier to be a complete asshole (less stress, less time spent explaining things to people, more time to do things you have to do), hence why a lot of managers are assholes.