In this New Avengers 32, it is said that the Beyonders are in a solid state, a manifestation from their essence. They are beyond any comprehension (like Pym said). By this they assume the form familiar to those who are communicating with them (mechanical form).
I assume their physical form is weaker than their true essence (maybe like Onslaught physical and essence form).
Their deaths are meaningless, they didnt kill any of them in reality.