Knowing that he gets assassinated makes this book/story difficult to read.
It's difficult to get through this book/story because I know he gets assassinated.
「彼が暗殺されると分かっていて物語を読み進めるのは辛い」は英語で Knowing that he gets assassinated makes this book/story difficult to read もしくは It's difficult to get through this book/story because I know (that) he gets assassinated になると思います。
暗殺する - To assassinate
物語 - story, tale etc. eg 源氏物語 - The Tale of Genji
読み進める - to continue reading
辛い - difficult, tough etc.
イギリス 
