You may have grown up with the belief that being angry, showing anger etc are all "bad" things. But anger is an important emotion - and it happens in all of us - so when it happens, it has to "go" somewhere.
With some people it comes out without control. It is hurtful and at times physically and emotionally damaging for those on the receiving end - as some of us well know and many of us have, at the very least, observed.
With others it is expressed in safe ways and controlled. This is usually a learned behaviour - we have it modelled for us by parents and other adults, we learn as children to express it in safe ways and we make those choices as children and adults as to how we deal with those emotions - just like we do with all the other emotions we experience.
Then there are those of us who turn that anger in upon ourselves. We don't know how to express it safely - we lack the role models or the know-now to do it in a way we feel OK about - so we keep it inside. But it doesn't die in there, it starts to get expressed in other ways ... it leaks out in an uncontrolled way or ways. Then we feel bad about it, we beat ourselves up about our behaviour (especially if we believe that anger is bad) and this judgement in turn hurts our self-esteem, our confidence and our trust in ourselves. It becomes a very damaging cycle and can turn be expressed via self-harm or suicidal behaviours - based on a very strong belief that "we" are "bad".
Which one are you?