There are no answers here. The truth is you, and the entirety of the Czech Republic were grievously injured in your collective soul. Whatever the motives of the shooter was, it is a senseless crime whose goal was to paralyze your heart.
Grieve first. It's necessary. However, don't let this cruel, twisted soul poison you. They win if you let it happen. Use your pain. Start by being with friends, classmates and family. Offer little kindnesses. A cooked meal or a timely errand, even pausing to listen to another person who is working through the same pain you are. It's doesn't seem like much, but it is your humanity that will get you through this.
When you finally begin to not feel so frozen, maybe invest time into social action to prevent this from happening in your country again. Work to be sure our American disease doesn't get a hold of your body. My country is broken here and the lesson you need from us is not let it infect you. Fight for it if you can.
Again, my condolences for all of your losses. They're probably inadequate, but know you are not alone.
Take care, stranger.