The atheist misunderstood.
There are basically two things here: (1) Grieving for the loved ones departed and, (2) Christian belief of an afterlife eternal.
They are not the same.
The first, feeling sad for a loved one departed is normal. It would be abnormal not to feel sad as the deceased would be certainly missed. Even Jesus cried when his good friend lazarus died.
In the Sutherland Springs case, it should be even more, as the killing was cruel and should not happen. It is insane to be happy about it, Christian or not.
Number two, we believe that we will have eternal life in heaven if we have lived our lives well on earth according to our belief and teaching of the Church. For that we look forward for the day when we will meet the Lord, not that we seek death because actually God want us to live the earthly life, but when the time comes for us to die, we will not be defeated by it. In that sense, yes, we are grateful for God’s salvation as everyone will has to die when their time comes.
So they are two things actually.