We know their pain has ended. We can also let them know it is OK, but we miss their physical presence terribly and wish they didn't have to leave. We would things could be different, but they are not. Truly we don't have to like what we acknowledge and accept. Our loss hurts, but that can be the witness to our love.
Anger is real and justifiable.
It is usually appropriate around these matters. Recognize, own, and be ready to let go of feelings that will only become harmful. When their appropriate usefulness becomes destructive then do what you can to process and clear. Replace negatives with positives. There are Angels to help.