That’s reductionist.

All people have morals; the ones to be fearful of are the ones that allow their morality to bend depending on the recipient.

Its a hueristic, its quite literally wrong but it is a quick and useful guide.

Useful in some contexts, disastrous in others. The challenge with all heuristics is having the wisdom to know when to apply them.

I think the self interest heuristic is generally overused.

I guess I disagree, self interest is generally underestimated to disastrous result. The reason being that if you assume someone is out only for themselves and are wrong you lose very little. If you assume the opposite wrongly it is very costly.

The way I see it we are fundamentally the same species which committed all of the atrocities of the 20th century, 19th century, heck all of the extremely violent and callous behavior in recorded history. To believe that human nature has changed much from that in only the span of a few decades is foolhardy.

How many wars and atrocities of the 20th century were caused by this heuristic as well?

If you assume most people are out for themselves, the rational solution is usually to violently suppress or exterminate your opposition whenever possible. Kill them before they have a chance to kill you.

it's not that simple. if i just assume that someone is out for themselves, and as a result i reject or ignore them then i'll never find any friends.

so i need to assume that most people are not just out for themselves and engage others hoping that they are not, but maybe just be cautious until i know them better.

Nope. Sociopaths don’t have morals. And most people will do immoral things if the value/cost is high enough.