My understanding if you read the Israeli news articles is that the justification is that the car was going fast:

https://www.timesofisrael.com/israeli-forces-kill-west-bank-...

Given that the IDF involved were undercover agents (according to the reports), it seems unlikely that this family knew that driving fast would get them killed.

From the article, an eyewitness account:

> He told us the family car had just turned left into his street, facing uphill, and had come to a complete halt before any shots were fired, contradicting the Israeli army account. I asked if he had heard any warnings given by the Israeli forces, or any warning shots fired. "No, nothing," he said. "The firing directly targeted the car. I just heard the woman in the car screaming. The little kids were crying before they were killed."