A lot of people in this sub don’t seem to understand what the definition of terrorism is. According to the Oxford English Dictionary, it is the use of violence against civilians for political aims. Hamas is a paramilitary organization, let’s just call it an army for ease of discussion. There is currently an actual shooting war going on (no cease fire) between Hamas and Israel. Members of an army’s leadership are legitimate targets during times of war.
Now, attacking an army’s leadership in a third country (I’m counting Gaza as country 1 and Israel as country 2 for this discussion) can have big, negative repercussions for the country that does the attacking in the 3rd country. However, this assumes country #3 is trying to remain neutral. Iran is anything but a neutral 3rd party in the conflict. They have armed and trained Hamas for decades as well as threatened to destroy Israel many, many times over 40 years. For Israel, there was no real downside to killing that Hamas leader in Iran.