I don't think anyone is suggesting that Hamas is not engaged in terrorist activities. However Hamas would argue that these are reprisals for terrorist activities carried out by Israeli forces. The same logic the Israeli authorities use when they justify killing innocent people, including a four-month old baby, in reprisal raids. "Collateral damage" as Timothy McVeigh would call it.

I do not think that responding to acts of terrorist by acts of further terrorist is ever justified. It also doesn't work, if the aim is to stop the other side reacting violently.

And in fact I do not think it is intended to achieve that, whatever is claimed by apologists for the killers on both sides. The aim of reprisals is in fact often to ensure that there are counter reprisals, and in this way to keep the pot boiling, and squeeze out people on your own side who might try to make peace with the enemy.