An aphorism is always considered a general truth in the long run even when the primary meaning may be meaningless there is still always a compound truth when it comes to aphorism . the sentence it's quite complex on the general surface so before you make meaning of it we also have to consider breaking it down in order to get it simplified, how about also looking at the literal meaning?
Truth is that not everyone who wields a sword in physical will actually be killed with a sword , the cognitive meaning of this aphorism is that living dangerously will also warrant a dangerous end to one's life, for example someone who is actually the criminal and evading the hands of the law robbing people of their property and money it means that during the end this particular individual will not likely die by being buried peacefully as a result of a natural death he will actually die by either being taken to prison or giving jungle justice or also killed by firing squad which of course related to what he did.
So the aphorism in normal sense is actually saying things happens synonymously and the life which one lived will actually have to relate to the way which they also die also, so yes I believe in it so much because the lateral meaning may seem so confusing but in the actual, logical and literal meaning it actually makes a lot of sense in the long run.
So it generally does not say that someone who is a swordsman would only be killed by a sword of a competitor but however he may also have a synonymous death to being killed by sword, an axe or a spear which looks related, this is 70% supposedly true. So yes it is actually very true and I believe it, you much know that aphorisms are always confusing but He who kills by the sword must die by the sword" is logically and literary true and it's essence is so rich when considered.
Qui gladio occidit, gladio occisus erit
(He who takes the sword, with the sword will die)
To receive as payment, the same currency, that is to say to experience in our flesh the same damage that we cause to another. The phrase appeals to divine justice, which indicates that sooner or later the violent will receive the same treatment as they give to others.
One story says that one of those with Jesus drew his sword and wounded the High Priest's servant, cutting off his ear. Jesus said to him, "Put away your sword, for he who kills with the sword must die with the sword.
And it is that generally, one receives or experiences the same damage he did to another. I want to come to believe that it is completely true, because it is like a kind of "karma" to be afraid of.