James I think it's good that you're willing to express your doubts and concerns - so you shouldn't be scolded for it. Like Cr4w say below, it's hard to appreciate when you aren't "othered" or aren't part of the dominant group in society. In terms of Alex taking on responsibility, I think a lot of that comes down to how people make both conscious and unconscious associations when they have an experience and how their previously established biases affect those associations.
As a simple example consider someone who makes their first visit to a fairly unique ethnic restaurant that is their first interaction with that type of food. If their experience is good they will be more likely to consider that type of food as being good and be interested in eating it again. However, if their experience is not good they may be more likely to consider that type of food as not good and not be interested in eating it again. The problem is that they are making a judgement of an entire style of food based on one unique interaction that has many variables that determines whether the food was good. Their single and unique experience from that one occasion should not serve as a reference to that type of food in general.
In a similar manner, Alex has to consider how his interactions with people on the trail may influence their future perceptions with people that look like him. The fact that he is an imposing figure due to his physical size only heightens that consideration. He has to take greater consideration for his actions simply because he is unique and people are more likely to associate their experience with him due to his physical uniqueness rather than his whole identity as a person.