There are many sad death scenes in the film but out of all of them, the death of Tsu'tey was the saddest for me. He is lying there bleeding to death with several Na'vi survivors and Jake and Neytiri at his side. He finally expresses his respect for Jake from the bottom of his heart, referring to him as a brother and passing on his leadership to Jake. the "last shadow" slowly covering Tsu'tey before he is put out of his misery by Jake and the music made the scene a touching moment and the best addition that came out of either the August re-release and the Collector's edition, in my opinion.
On the other hand, Trudy's death was sad but pivotal to the outcome of the battle. Had she not intervened when Quaritch's Dragon gunship was pursuing Jake, he would have likely died right then and there and Quaritch would've won, even with Eywa's intervention since none of the other Na'vi warriors would have been able to take out either the Valkyrie or Dragon as they had no experience in handling grenades and explosives like Jake.
Eytukan's demise happened at the most unfortunate of times. When he entered Hometree, now on fire and completely ruined, to save the Na'vi who were still inside from the flames, thinking that the attack was over and not realising that the RDA were about to switch to rocket explosives and open fire on the Hometree's collumns, gets impaled on a piece of wood. The reason why the timing was so unfortunate was because he died believing Jake was truly on the RDA's side and a traitor to them all, and did not live to witness Jake's redemption, which really sucks.
Grace's passing was emotional for me because Jake spent so much time to ensure her survival, giving her medication and getting the Omaticaya to save her, only for her to die anyway, though it was somewhat comforting to know that her soul became one with Eywa.
Like most people, I was happy to see Quaritch killed in the end. I know there are some who defend him, claiming that he was merely doing his job, which was to destroy Hometree, and that is true, it was his job, but the fact that he didn't allow the Omaticaya to get out of there first after they realised they couldn't stop Quaritch's fleet, and didn't order a chopper to land and cut Jake and Grace's Avatars free before giving the order to "bring it down" makes him one cold blooded bad guy. Punching a crippled person who can't adequately defend himself and exposing a room full of people to the Pandoran air without giving them any time to find a mask first doesn't help in making him a likeable individual either.