They may not care about you, but your atoms are useful to them in their current configuration.

There are ways of hurting people other than stabbing them, I just used a simple example.

I think there is a confusion about what exactly "selfish" means, and I blame Ayn Rand for it. The heroes in her novels are given the label "selfish" because they do not care about possibilities to actively do something good for other people unless there is also some profit for them (which is what a person with zero value for others in their preference function would do), but at the same time they avoid actively harming other people in ways that could bring them some profit (which is not what a perfectly selfish person would do).

As a result, we get quite unrealistic characters who on one hand are described as rational profit maximizers who don't care about others (except instrumentally), but on the other hand they follow an independently reinvented deontological framework that seems like designed by someone who actually cares about other people but is in deep denial about it (i.e. Ayn Rand).

A truly selfish person (someone who truly does not care about others) would hurt others in situations where doing so is profitable (including second-order effects). A truly selfish person would not arbitrarily invent a deontological code against hurting other people, because such code is merely a rationalization invented by someone who already has an emotional reason not to hurt other people but wants to pretend that instead this is a logical conclusion derived from first principles.

Interacting with a psychopath with likely get you hurt. It will likely not get you killed, because some other way of hurting you has a better risk:benefit profile. Perhaps the most profitable way is to scam you of some money and use you to get introduced to your friends. Only once in a while a situation will arise when raping someone is sufficiently safe, or killing someone is extremely profitable, e.g. because that person stands in a way of a grand business.