In the beginning of chapter 5 Dawkins explains how survival machines and their genes constantly compete or benefit from their environment. He states that all the other survival machines in the environment, whether they being from the same species or from another, as a survival machine containing the selfish genes, you may either benefit from these other machines. Clear examples of benefits are seen in altruistic behaviour.
As humans, our life revolves around relationships, especially social ones. We have parent-child, friend, love, hate, teacher-student, etc. In all of these relationships, if we truly observe them, we realize that there is always one survival machine gaining something from it or at times all the machines involved may be gaining something.
The funny thing about what Dawkins says about using other machines for your own purposes is that this is completely true. Throughout our entire lives we are taught that we must share and be 'good' people. As if to say, we should have teamwork skills and never 'walk over' anyone to achieve your own goals. But this is basically impossible, not only is it in our nature but current world circumstances oblige us to act like that.
For example, if you are working in a company and you would like to be ascended, you only have 2 options of doing so. Either waiting for your 'big break' which at times could take up your whole career life, slowly ascend your way up the ladder, or you may use your colleagues as 'steps'. As if to say, benefit from them. Globalization today has taught us that the most important thing you need to do in order to succeed in this world, is ambition. Hitler, Mussolini, Ernesto Guevara, etc. All of these great leaders had it. They may not have used their power for the best purposes but they had ambition. And this was their fuel to achieve the great things they did, all the changes they created.
So aggression is in our nature, but in our current situation aggression may be the only way to be the last survival machine standing.