Why use the negative ない in 自分のことしか考えてない男のはずだ


I saw this sentence in an episode of Attack on Titan

A character that is believed to be selfish does something perceived to be
selfless by the the speaker, and the speaker says

> 自分のことしか考えてない男のはずだ

which was translated in the subs to mean

> The Jean I know is someone who only thinks about himself.

Why use the negative 考えてない in this case? Wouldn't that mean he doesn't only
think of himself? How is it different from using 考えてる or 考える?

