S-300 is not a "killer rocket" that Israel uses against civialian populations. It is a an air defence (not an offence) system with a high-kill probability of 90% to protect airspace intruders and keep the local population and objects safe (almost like the US patriot system). Read more on wiki.
Donald Trump: Israeli settlement construction in West Bank should 'keep going' (Haaretz)
from the article: Iran official: We have obtained the S-300 missile system