For understanding, if anger is a sign of strength or not, we need to first analyze when and why somebody gets angry.
Simply put, You could get angry when things are not going 'your way'. It could be anything from a personal/professional feud to a lack of respect shown towards you or even a non-person matter like a messy kitchen. These situations make you raise your voice and question or confront it. In doing so, you can feel some empowerment coming over and you can feel very strong. Getting angry when threatened or challenged by a situation is natural and normal.
However, Life-threatening situations aside, the fact that something outside of your self is perceived as a threat is suggestive of underlying insecurity and vulnerability. In today's modern world reality, most of the provocations are not related to physical harm but only pose a threat to our ego. So, getting aggressive suggests not a resilient ego but rather a fragile one.
A very typical instance of receiving criticism or evaluation can thus make you angry because you feel the need to nullify the authority of the negative appraisal. Or in other words, you are unable to grant to somebody else the authority to criticize or concede to their viewpoint. So, almost as an act of self-protection, you retaliate to nullify that authority. And like mentioned earlier, the anger (and the subsequent empowerment) that you feel, makes up for the feelings of “powerlessness” in you.
Conversely, the more secure your self-image is, the calmer, more level-head and open you will be.
Anger, in a lot of ways, shows a lack of strength to stay vulnerable and open to responsive situations. It is also like a “defense mechanism” or a “bluff” to not expose any insecurity or vulnerability underneath.
For example, telling somebody your reservations about your presentation at work would be to openly admit your vulnerability and be transparent. In contrast, criticizing people who give critical feedback on the presentation so that they never find out about your self-doubt seems to be the 'stronger' thing to do.
So at the end of the day, being angry and displaying your anger is indeed like a shield to protect you from anxiety, self-doubt, inferiority complex, etc. Not facing your self and confronting your emotions, is the textbook definition of being weak.
Strength comes from within. When you are complete and whole, You are comfortable with listening to feedback. The 'victory' that you feel after an angry episode is only momentarily and just as the anger begins to fade, you are often filled with conflicting emotions of embarrassment, reproach, self-doubt, and “defensive pride”.
Being angry and choosing to remain angry, hinders your personal growth, and does not let you reach your level of self-actualization. That in turn, reflects on your professional and social life so you have no fulfillment there as well. You are left with a skeleton of what could have been. So, choose yourself and your growth and learn ways to express anger healthily.