Two moments of self-sacrifice in the second year of TOS really stand out for me...
1: my all time favorite ep is "Bread and Circuses". Even though we all want to see Kirk take a shot at Proconsul Claudius Markus during the blackout in the arena, or at least slug First Citizen Merrickus, the shocking ending occurs when Captain Merrick lays down his own life to signal the Enterprise to beam up Kirk. "Starship? Lock in on this... Three to beam u--...!" followed of course by the jaw-dropping sound of Scott's all-business reply: "Enterprise to Captain Kirk. Ready to beam up three."
2: And another scene of daring, played to virtually identical music, was in the final act of "Friday's Child", when newly crowned High Te'er Ma'ab confers with Ele'an and K'eel, then goes out and offers himself for Klingon target practice so Ke'el can get a clean shot on the Kraas. Talk about going out on a blaze of glory... "KLING-ONNNNNN!"
BTW: I also love the scene where Kirk gives the word and Spock finally relives Decker. Great lines there! "Blast regulations! ...I order to to assume command on my personal authority as captain of the Enterprise!" "I don't recognize your authority to relieve me." Followed by the only-on-TOS line "You may file a formal protest with Starfleet Command, assuming we survive to reach a starbase."