This Chinese idiom sounds wrong, but if so, then it wouldn't be one.
Logically speaking, it is correct, or true.
However, in most cases, we prefer the then to be false. For e.g., if you don't study, you will fail the exam. Thus, that makes not studying a bad choice.
On the other hand, consider this: if we don't punish children, they will not learn. Here, is punishing good or bad? Many would think obviously it is good. Actually, this depends ... on what they will be learning?
Thus, whether 人不为己 is good or bad depends on if 天诛地灭 is good or bad, or if the current "天" and "地" are good or bad. If these are bad, then 天诛地灭 would be good, and therefore 人不为己 would be good.
I think, instead of sustaining a world where 人不为己 is bad, isn't destroying, or actually transforming such kind of world into one whereby everyone is 人不为己, better?