"How to make a character's death sadder"

1. Don't have them die of old age after a long, fulfilling life. Many people don't even think of this as sad (not that this can still work if you have enough of the other factors).
2. Leave one of their major goals unfinished. The more enthusiastic they are about completing the goal, the sadder.
3. Give them strong relationships with other characters.
4. Make them fight against whatever is causing their death. Their ultimate loss is sadder if they struggle.
5. Kill them in the middle of their character arc.
6. Don't describe their funeral in detail.