many others have answered very well to the main topic of the thread, i would like to comment only on this question which you two raised…
Qatl-e-murtad is part of Islam. when someone first enters Islam, he is supposed to accept the entire religion. s/he takes oath to follow Islam, which includes adherence to all the laws/ rules. so they accept that if we now leave Islam, punishment will be to get killed.
now, how this punishment (qatl-e-murtad and others eg: qat’a yadain etc) will be given (or not) is a different story, and different schools of thought have different opinion on it, which we are not discussing here.
and one important thing, do not single out Islam in qatl-e-murtad, because other religion(s), such as Judaism also have this punishment in their shariah