So in short, Kiritsugu had a really cute nickname, lived in an island with his dad who starts an experiment which turned everyone into vampires/zombies. Feels very BLOOD-C/Shiki/Higurashi which really shows why Kiritsugu is so screwed up currently.
The last time I saw a back story this good was Kamisama Dolls. It really gives Kiritsugu a new face. You get to know him more and how his ideals started to slowly form to what it is now. Fate/Zero really took advantage of the whole episode, even going as far to skip the OP.
It’s hard not to gush about this series. It really is a story that’s executed perfectly with the right amount of story telling, narrative and dialogue without having to baby feed you too much. At the same time it presents ideals and themes that really questions the topic being tackled to the point that even I’m starting to question myself.