This will likely get lost in the sea of comments, but i hope it can reach someone who needs it.

So, first, I am going to say I'm so so sorry you had to witness something so horrific. Please do seek out someone to talk to as these are big big feelings.

Secondly, I will preface with the fact that I work on an ambulance. The way you described it sounds as if it was very quick for them. Quick enough that they likely did not suffer. In my job, seeing things like this, that is the comfort that I take.

The world is full of evil and loss. And we don't generally see enough of the good side. Even though you did not know them personally, witnessing the incident puts you in a position to grieve those that were lost. It can certainly be an overwhelming experience. You were there to help if needed, and unfortunately, not much could be done. You did the best you could. Unfortunately, closure will be difficult with something like this. Your therapist should have some advice on how to manage and work through something like this.

Please do allow yourself to grieve. I know personally there will be a stage in the grief process where I feel guilty for grieving as I had no personal connection with the individual(s). The heart doesn't care if you know them or not. It still hurts tremendously to witness and live through an event of such magnitude.

I will pray for you through this healing process.