The scene towards the end of Behemoth, where Alek begs Dylan to leave the Leviathan behind as he doesn't want to go without him. Dylan convinces him to come back onto the Leviathan to be a political prisoner.
Alek giving up his secret - that he is the heir to the throne - to keep Deryn safe.
When Alek kills Tesla, he doesn't do it to save the millions in Berlin, he does it to save Deryn.
And finally, Alek giving up his claim to the throne so that he can be with Deryn. The things he does for her....