Out-of-context, the intro movie of Nicole and Isaac is quite sweet. You can tell how much they love each other.
Now, the Nicole hallucinations are agreed to be terrifying. Then you get the one in the mines. Nicole is slamming Isaac against the walls, screaming at him, asking why he won't let her go. Hitting the button prompt leads to Isaac admitting "Because you were my everything. And if I let you go, I have nothing left." Nicole immediately drops him and turns into normal Nicole. Then, in this calm, touching voice, says; "Step Four: Acceptance. Now you're ready to finish this."Sorry,I have something in my eye.
Equally, after Isaac sends Ellie away. He slowly collapses against the wall, crushed. The camera turns and, hey, it's non-demonic Nicole! She quietly asks if he wants to talk about it, like she genuinely cares.
It turns into a subversion when she reveals that she only turned good to manipulate you further.
It manages to simultaneously be really sweet, really sad, and really creepy when she asks Isaac to touch her and "make us whole".
This Troper thought that the only thing more heart-warming than Isaac staying behind so Ellie could escape was watching Isaac collapse at the end, only to be greeted by Ellie smashing in through the roof to return the favor (even after she discovered that Isaac caused the outbreak.)
The Mythology Gag ending. After going through hell Isaac escapes with Ellie. He turns around, after settling into the cockpit of the gunship, and the music scratches. Instead of seeing Nicole as a necromorph, he sees Ellie, who says "...What?"
Whilst it may not seem like much, discovering that Tiedemann, despite being the villain, disobeyed orders and helped a large amount of people escape kinda reminds you that he's just following orders.
In Severed: The ending of Severed, in a weird way. If you've played Extraction, you'd remember that Gabe was kind of a dick, but his final moments are pretty heart-warming in a way, you can see how he's changed from the man that he was.
In Dead Space 2, after Ellie finds the gunship and is really glad that, hey, they're going to survive, Isaac looks at her and simply says "One of us will" before locking the controls on the gunship and detaching it from the station. Ellie is in tears, and Isaac's not taking it well either, but the fact that he's going so far to ensure her safety, after admitting that he simply can't let go of Nicole, is surprisingly powerful.
There is something touching about the fact that, at one point in the game, a survivor left a log telling about how he discovered a way to use the alien's own arm-blades as projectiles. It's also the last thing he does, because he's bleeding out. You find the helpful video near his corpse. In a series where people decide to spend their last moments killing themselves or actively trying to make the situation worse, at least one person's dying act was to try and help others survive the ordeal.
In 3, Isaac even shows this log to the rest of his party; he remembered the security officer he never met who used the last moments of his life demonstrating a useful survival tip.