Everybody has it in their own mind what is acceptable and what is not. Some things might be mostly standard, for instance we don’t approve of someone who physically hurts us. Other things might be more subjective, maybe not even rational. When boundaries are crossed, anger is a frequent reaction. This is totally acceptable, we are allowed to feel anger at any time and it is a natural reaction when boundaries are crossed.
As an invitation, the next time you see that someone is angry, I invite you to truly make sure you understand which boundary you have crossed. Sometimes this is obvious, but other times this is much tougher to distinguish. If this is part of a relationship which means a lot to you, then you owe it to both of you to know which boundaries exist, and where. By identifying the boundaries, the relationship becomes much more sustainable.
We all have boundaries. Find out what they are, if possible before they are crossed. Accept people as they are and don’t try to change them for someone else.