No, papal encyclicals are not "part of the magisterium."  The magisterium is the teaching authority of the Church.  Just because the pope or a bishop expresses an opinion in writing doesn't mean he is exercising the teaching authority entrusted to him.  The dogmatic council of Trent, which certainly did exercise such authority (and fully intended to), and which was promulgated by a pope who fully intended to do the same, said that the state had a duty to punish criminals including by way of the death penalty for crimes deserving of such.
Why would you believe that Evangelium Vitae had any more moral weight than Casti Conubii?  They are both encyclicals.  One teaches the opposite of the other.  But in the case of EV, it teaches something in stark contradiction to the dogmatic council of Trent.  JP2 was clearly on the opposite side of Church's millennial and traditional teaching with EV, as is Francis.