Growing up in ministry I have had to attend quite a few funerals. I've also lost to the specter of death several children and both of my parents and a dear mother-in-law. So, I've experienced and also overheard some of the horrible things people say at the time of a death in an attempt to comfort the bereaved. "I'm sorry for your loss", is very often used and though it's meant to be kind, the hundredth time you hear it raises enough anger in the bereaved to want to lash out. The thought comes strongly, "you have no connection to my loss or what it is costing me so just get out of my face." You could say, "What a terrible, terrible loss. I'm so, so sorry" and give a warm hug. This better connects to the grieving heart.
Another statement that really causes hurt and raises my ire is, "God needed another angel in Heaven." This statement blames God for the death, yet the Word of God states that death is God's enemy which He will one day destroy.
The Word also says that "Precious in the sight of the Lord is the death of His saints." God knows exactly what it feels like to lose a child in death and Psalm 23 says He walks through the dark Valley of Death with us. He weeps with us, He feels the loss for us. And besides this, angels and humans are completely separate creatures. One does not turn into the other. In fact, the book of Revelation states that humans will sing songs the angels can't even learn. You might try saying, "He had a very special purpose on this earth and he completed his job well. He's going to love his reward."
People often say, " If I can do anything for you let me know." Again, they are well meaning but ignorant of what the grieved is going through. The bereaved feels frozen into a nightmare they can't wake-up from. They don't know what they need. So, if you really mean what you're saying then don't wait to be asked. Drop by and mow the lawn. Drop in, fix them a cup of coffee, set them at the table while you do up the dishes, mop the kitchen floor or throw in a load of laundry. While you are doing that, talk of sweet or funny memories you have of the one who is now gone. And LISTEN! LISTEN! LISTEN! to them. Memories remind the bereaved that they haven't lost everything.
You can also acknowledge that they are going through a long dark tunnel right now but Jesus is right there with them and will put a torch on the wall every once in a while to let them know He's just up ahead. Remind them to look for the torches and to soak in the light for a bit before going on in the tunnel. Visit them regularly and be very practical in the needs you see and meet. Ask them if they have seen any torches this week (a pretty sunrise or sunset, a bird's song, someone being nice in the grocery store, a smile from a small child, etc.) and revel in the light with them.
Death makes us all uneasy and we try to erase it's painful effects as quickly as possible but that only makes the bereaved feel even more frozen and lonely. Walking through their loneliness, heartache and hopelessness with them is good for them and for you. God did not bring death into this world. Two disobedient humans did. But God can use it in all of our lives to give us a closer, loving walk with Him and a greater depth of character and empathy towards others.