At some point everyone who wants something becomes an enemy. The question is when and that is so dependent on many local factors that just common sense and good judgement can help to answer this.
Good point, and very important one.
People often forget that everyone outside your family-group can ( will eventually) become enemy. It is only matter of time and seriousness of SHTF event.
Few days may look like camping, people trying to help each other, but after few weeks or months when resources become rare rules change.
Your neighbor who was your good friend would like to see you dead much more then to see his kid dead.
It is choice.