I sometimes think that beginners of martial arts may become more aggressive, IF they are the type who is learning for ego purposes, to be the tough guy / gal, etc. Usually these people don't last long, fortunately. Once one is an experienced practitioner, with correct training, however, one can become acutely aware of the damage that one is able to inflict in a few seconds or less, and thus be extremely wary of misuse. A good practitioner will be able to exert enough technique to quell the situation, but no further. Beginners cannot, and tend to use all their power, which can be catastrophic. Moreover, long term practitioners are used to the situation, and thus seldom arise emotions of either fear or anger; again, beginners do not, and easily get angered. It's quite complex. Once a practitioner is a real master, then they'll have different mental states to this, too. Their mind-body connection is incredibly strong and powerful, and this leads to deep states of concentration even whilst active.
My recently moved Blog, containing some of my writings on the Buddha Dhamma, as well as a number of translations from classical Buddhist texts and modern authors, liturgy, etc.: .