A friend who betrays you in the end was never your friend to begin with.

问题描述:

A friend who betrays you in the end was never your friend to begin with.
这里的to begin with的用法我有些不明白,

不定式作定语
可以等价于.your friend who you begin with.
用两句定语从句显得累赘,所以用不定式代替