There are a lot of characters in the Mass Effect series; some make severe impacts on the gameplay and storyline for gamers. They advance the narrative and story, and with some, it is tough if and when they die on screen. Even if the player manages to save themselves or the world at the end of the trilogy, there are a lot of deaths that are not preventable.
If the player chose to take the Renegade path or just didn't care enough to gain follower loyalty, there are a lot of extra deaths that can happen, but for now, here are the top ten worst unpreventable deaths from the Mass Effect games.
And this goes without warning, but spoilers ahead.
10 Navigator Charles Pressly
Pressly was the first person Shepard can choose to talk to in the first game. He wasn't the most significant character, and he was pretty xenophobic toward the Turians, but his death in Mass Effect 2 when the first Normandy was destroyed did hurt. The player can talk to him a bit in the first game while exploring the Normandy, and he was a good man to have aboard the ship. It must have been harder for Joker, who had served with him for a while before Shepard came along and saw him die before his entire ship was blown up.
9 The Child
This kid was the bane of Shepard's existence in Mass Effect 3. After watching him climb aboard a shuttle that promptly gets destroyed by a Reaper, then having him haunt their dreams for so long, just for him to show up as The Catalyst right before the worst decision in the game has to be made. It makes this unnamed character's death stretched out and heartbreaking.
Since no one knows where his parents were, and he decided not to go with Shepard, it could be safe to assume he was an orphan. Either way, Shepard felt too much guilt over his death.
8 Alec Ryder
Andromeda has been known as the Mass Effect killer. And while the game is not nearly as amazing as the original trilogy was, it doesn't make Alec's death much more manageable. The father who gave his life to save his child. It doesn't help that Ryder woke up to find out that their father died to save them. They are also traumatized and haunted by their dead father and manage to learn all that he's done later in the game. Drags the grief out and hits the player with a lot of feelings.
7 Saren Arterius
For the majority of the first game, The Council seems to think that Saren is the mastermind behind all the Geth attacks. Of course, Shepard knows better that the Reapers have been controlling Saren to do their bidding. It makes Saren's story and ultimate death a lot harder to swallow.
Saren genuinely thought he was doing a good thing. He knew the Reaper's were coming, and thought he was doing all he could to make sure as many people survived as possible. Sure, he was the bad guy, but the player had to feel a little bit sad when he tried to blow his head off before the final battle.
6 Legion
Legion is the first friendly Geth that Shepard meets. Legion makes it clear that all It wants to do is somehow find peace for the Geth. Legion shows a more human side to the sentient droids that had been pursuing Shepard for so long. Shepard can begin to see them as indoctrinated by the Reapers as the other species could be. Legion's death is one of the many previous follower deaths that Shepard can't prevent. Whether Paragon or Renegade, whether they chose the Geth or the Quarians (or managed to save both), Legion has to die. It is probably the only time the player felt sad about a Geth dying.
5 Thane Krios
Thane is one of the only romance options whose death the player can't prevent. There are different ways for him to die, but the longest he will survive is gaining his allegiance in the second game and talking to him at the hospital in the third. Thane ends up dying a hero, saving a councilor. It is especially painful for a female Shepard who romanced him, but the entire death scene is ugly-cry inducing. If the DLC Citadel is played, after his death, there is an emotional scene with his memorial where his son will reach out to Shepard. Overall, Thane is a harsh death to get over.
4 Kaidan Alenko or Ashley Jenkins
There are multiple ways these characters can die, especially if Renegade or Paragon points are not high enough. Shepard is forced to leave one of their human companions behind on Virmire in the first game with a bomb about to go off. After the mission, the one you saved feels guilty (and makes the player feel guilty) that Shepard saved them. The character the player leaves behind is brought up so much throughout the series that it is hard to get over that death.
Then, of course, halfway through the third game, Shepard is gun to gun with the one they saved three years prior. It is very possible for them to die too, the player has to be really careful if they didn't romance them.
3 Mordin Solus
Unlike Thane, who knew his death was imminent, Mordin's sacrifice was his way of atoning for a past mistake. He was part of the team of scientists who created the Genophage (the genetically modified plague that forced infertility upon female Krogan). He was fine with his decision throughout a lot of the second game. It was an especially sad death because the player watched the entirety of Mordin's character arc: going from thinking his decision was the best made at the time, to realizing his error and devising a plan and putting it into action to correct it. He regretted what he did, but he did not live in it, he made up for it. Of course, his singing during the scene was uncalled for.
2 Admiral David Anderson
Admiral Anderson had been with Shepard through the entire series. He gives Shepard his ship, his apartment on the Citadel, they learn so much about him throughout the series. He was like a father to them, constant in advice and wisdom, his death is second to last that the player sees before the ending of the game, and arguably the most heart wrenching. He tells Shepard he's proud of them, and that just feels like the player's father saying it, right before he bleeds out. Throughout the whole third game, people were dreading getting the comm or email saying that Anderson had been MIA. Just for him to be shot, then die right in from of them.
1 Commander Shepard
Shepard's first death was very painful to watch. The player hears them gasping for air, and on their first playthrough is thinking "what just happened?" Not to mention all their friends, teammates, and possibly love interest now having to grieve them.
And then Shepard has to go and die again!
So, yes, there is one way for Shepard to live in the end. If the player has enough military support and chooses the "destroy" option, in the final cutscene, Shepard is seen slowly climbing out of the destroyed Citadel. But the more righteous way for the game to end is with the "synthesis" ending, where no one dies except Shepard, and everyone maintains free will. It is heartbreaking to hear the epilogue, for more reasons than Shepard dying. It's the end of a long journey, and the player is sad that there's nothing left. It was not the ending anyone expected or wanted.