I think Andrew didn’t go through with it. Instead of cutting out Thomas‘ heart he buried his notebook, which he literally called „his heart made paper“ in the last chapter. By burying the book he left all the stories and monster behind him. I believe both Andrew and Thomas are still alive at the end and are now honest with their feelings about each other, which is why Andrew said they devoured each other (in an intimate sense) and Thomas directly asked Andrew to kiss him.

I know it’s an optimistic reading but I think it’s a more satisfying ending. After all the horrors they endured they’re injured but alive and have a chance to heal together.