Because they are at war. And yes, it's an official war. Hezbollah declared war well prior to this and has been firing rockets into Israel. Hence, this is a retaliatory attack by Israel. It's no different than bombing or firing missiles, it's just far more targeted at Hezbollah members since it's their electronics. Unfortunately innocent civilian casualties are always a part of war, but this type of attack actually minimizes them far more than just straight bombing would.
The US State Department designated Hezballah as a foreign terrorist organization in October 1997. More than 60 other countries and organizations, including the EU, the Arab League, and the Gulf Cooperation Council, have also designated Hizballah—either in part or in its entirety—as a terrorist group.
Yahya Ayyash nicknamed "The Engineer" was a Palestinian terrorist, one of the founders of Hamas's Ezz ad-Din al-Qassam Brigades And one of those responsible for introducing the method of suicide bombings in the 90s as part of Palestinian terrorism in Israel. These terrorist attacks claimed the lives of over a hundred Israelis. Israel Shin bet Service assassinated Eish using a booby-trapped cell phone in 1996.