Don’t have them die of old age after a long, fulfilling life. Many people don’t even think of this as sad (note that this can still work if you have enough of the other factors).

 Leave one of their major goals unfinished. The more enthusiastic they are about completing the goal, the sadder.

 Give them strong relationships with other characters.

 Make them fight against whatever is causing their death. Their ultimate loss is sadder if they struggle.

 Kill them in the middle of their character arc.

10. have their loved one, broken hearted, tell the team to stop fighting because “its over.” 

11. have their pet come looking for them. 

12. have their loved one perform a popular song at their funeral so it makes the fans cry whenever it’s played.

13. family witnessing the death and/or blaming themselves.

14. kill the character in the middle of making a joke, smiling, or expressing/experiencing joy/happiness.

15. make the character’s death slow and painful, but make them unable to call out for help even though they can literally see the other characters nearby.

16. after killing the character, have others think the character had betrayed them so they’d always hate them and remember them as traitors and never say nice things about them… Give your readers no chance to have group-therapy with other characters by making them the only ones who know the truth.

17. right before their death, show a side of them nobody has seen. (someone who is always tough and brave being genuinely scared of dying alone; someone who is always laughing being in tears before dying, etc.)

18. make them the only person who knows a big important secret that would help other characters in the story.

19. have them being lied to before dying. (thinking they’ve been betrayed; thinking they weren’t loved; thinking they’ve lost their loved ones, etc)

20. make the character very enthusiastic/passionate about a certain goal, constantly put stress on their goal, have them die unexpectedly before they can reach their goal. 

and the best one…

21. have another beloved character kill them–better be a close friend to your character, one that absolutely nobody suspects, one that everyone can’t help but love, one who is always enthusiastic about things and encourages your character. THEN

reveal the truth only later when it’s too late and the a-hole character has already escaped.

have a cowardly character know the truth and never tell anyone else

have another character find out the truth and have them die before revealing it to others. 

have the said character ^ not actually die, but go through something so they’d forget the friend of the deceased character is actually an asshole. 

This way only your readers will know the truth, thus the frustration would be… most enjoyable for you.