The end scene where the prawn Wikus makes little metal flowers for Tania out of trash and leaves them on her doorstep.
Look carefully as Tania unwraps the flower: on the dresser behind her, there are images from an ultrasound and a framed photo of her holding a little girl. There's a good chance Wikus has a daughter he's never met.
It's also pretty hard not to feel bad for the poor guy when he promises his wife that he'll return to her.
The scene where Christopher's son asks when they'll be going home, and Christopher answers that they're not going now, and instead are going to the District 10 and even says how nice it probably is. Especially sad since Wikus, the audience, and Christopher himself know that it's really not.
The scene in which Wikus is tested upon by the MNU scientists. Especially the part when they're forcing him to test the guns, tasering him when he refuses, and then they bring out the live subject: an innocent, terrified alien. That Wikus promptly freaks out is one of the first signs that he's not a total asshole, but that just makes it worse.
Wikus' Last Stand to get Christopher back to his shuttle, which is also a Roaring Rampage of Revenge as the power armor falls apart around Wikus. The look on his face once the mini mecha finally dies and Koobus drags him out makes it clear that he expects to be taken back to MNU for vivisectioning, but is too beat down to care about anything but the fact that Christopher got to safety with his son. CMOA as well.
The look on Christopher Johnson's face as he sees a prawn's corpse on the MNU HQ having been operated on and tested.
Wikus after Christopher has reached the alien ship and it takes off. The battle's done, every goal reached, and Wikus breaks down sobbing.
In the beginning during the evictions, Christopher's friend is killed by the MNU. Christopher's son asks him where his friend is and he can only simply say he is gone while appearing frustrated.