I think the phrase is meant to tell us about the incompetent and not about violence (even though it reads as this is what violence is). I don't think it says anything about people to whom violence is a first or intermediary resort or whether violence is a good or bad thing. They may or may not be incompetent but might just like violence or be very expert at it. Some people are very competent at violence.
I think the phrase means anyone who goes to violence as their last refuge is incompetent. Only the incompetent would take refuge in violence and only as a last resort.
I don't think many people would regard Hitler as incompetent (at
least before 1942). He used violence and the threat of violence very
effectively but also demonstrated competence in other methods
(political intrigue and propaganda for example).
Pol Pot came to power through luck at every stage and was completely
incompetent at running a popular movement or country and took refuge
in violence and it was his last resort.
I think the original phrase by Samuel Johnson, Patriotism is the last refuge of a scoundrel, is meant in the same way. I don't think it is saying that patriotism is for scoundrels or that all scoundrels will resort to patriotism. Some people are patriotic and are not scoundrels. Some scoundrels are not patriotic. I think the phrase only means that only a scoundrel would take refuge in it and only as a last resort.
In the case of Johnson, it is, as I understand it, hearsay of a phrase without context so we can only speculate what it was intended to mean.
In the case of Asimov, he is certainly a very intelligent writer attributing this saying to a fictional character who is meant to be one of the most intelligent people in a galactic empire of multitillions (or more) of people. Asimov may not have put as much thought into it as we are here but, then again, he may have thought about it a lot more. I don't think he would be so unscientific as to think that incompetent people always resort to violence (or take refuge in it as a last resort). There are obviously plenty of incompetent people who do not (I would say resorting to surrender or running away is more common). That is why I think it probably has a more subtle and specific meaning: violence as a refuge - only incompetent people go there and only at last.