There is a large diversity of reasons that can trigger an aggressive reaction from a cat.
Most cats are aggressive only when they play, but there are traumatized cats which cannot and are not helped by their masters to pass over the negative past experiences and hence are manifesting a constant aggressive behavior.
An adult cat will be a lot harder to tame than a kitten. In order to educate and help it integrate, a lot of kindness and patience is needed but the results will be very rewarding.
In order to raise a gentle and balanced cat, the training must be done between 5-12 weeks of age when every abuse lives traces.
You need to be very careful about the cat bites because they are a lot more dangerous than the dog’s. In the claws and in the cat saliva, there are bacteria that can cause severe infections. If a cat scratches or bites you, disinfect the area and consult a doctor for the adequate treatment.
The cats are prey animals, and their hunting instincts are permanently manifesting, whether their masters like it or not.
When they are playing, especially the young ones tend to run after their masters, jumping on people’s legs and “attacking”.
This kind of attitude may look scary but it is simply the way the cat adapts its predator instinct to the environment it lives in. They are just acting as little “wild things” on a very small territory that is an apartment.