1. How credible is this source?
2. Seems like they were acting on sound intelligence
3. Sounds like they decided to deliberately throw caution to the wind and not risk themselves by firing on the car instead of arresting.
4. Upon finding out they've killed other people they've tried to cover it up
I think the fact there was sound reason to be cautious, does not excuse those involved of murder. They could have surrounded the car from a safe distance, ordered them all out, and if they did not comply shoot warning shots before shooting at the car. They came to take them out, which is why they were confident enough to get close to the car and trigger happy.
Bhai jee, how would you arrest someone with a suicide vest? It sounds like the protocol in these IS cases is to shoot first ask questions later. If I was faced with someone potentially wearing a suicide jacket, i might be inclined to do the same. That is not to say people with suicide jackets cannot be arrested, they can and have been arrested before, but the risk is very high as these terrorists have previously blown themselves up to avoid arrest; so you can understand why they might choose to throw caution to the wind (not that i advocate such an approach).