Yes, Israel's policy of targeted assassinations of dictators is justified, because Israel needs to be on a constant vigil to defend itself. There are many nations who want to kill Israel. It is acceptable for Israel to kill those who want to kill Israeli citizens, before the citizens are killed by these dictators.
It is tempting to get rid of an enemy by assassination. It could even be justified by saying: this is an evil person, we do the world a favor by killing him. However, by stooping to assassination of our enemies, we lower ourselves to their level. We become criminals ourselves, and we lose any moral high ground that we might have claimed in the past.