The explosion launched by Reiner and Armin failed to kill the man, and Eren is out for blood. The pair struggle to kill Eren even after he turns hundreds if not thousands into Titans, but Mikasa is able to get to her love in the end.

Who killed Eren Jaeger?

Eren once again proves to be the better fighter between the two, but Armin manages to immobilize him long enough for Mikasa to enter his Titan's mouth and and kill Eren by severing his head from the spine, before kissing him goodbye.

Did Armin or Mikasa kill Eren?

Mikasa killed Eren to stop The Rumbling. Her choice to do the right thing, even if it meant losing Eren, set Ymir's soul free and brought an end to the Power of Titans. She made her decision after Eren used Paths to tell her there was no other choice. Mikasa killing Eren also holds a lot of symbolic detail.