The pen is mightier because the pen can stop the sword, but a sword cant stop the pen.
If you kill someone, they will not be writing anything. Some else may write it, but it would not be the same.
ex: a war. signing a treaty will stop the war.
And stabbing an ambassador will start one.
The pen reaches more people at a time and it doesnt matter where those people are. Where as a sword can only kill people at a close distance.
You ascribe the results of using the pen to the pen, but fail to do so for the sword. If the consequences of the written word can be called the consequences of the pen, then the consequences of using the sword can be called the consequences of the sword. Thus, if you kill the right person, you can easily reach just as many people in just as wide-spread an area as you could with a pen.