As many who played The Beast Inside I also was annoyed that it does not have an official chapter select option. I started to fiddle with the savefiles and found out a very easy way to edit the savefiles so that you can start in whatever chapter you want. All you need is a hex editor (I used HexEdit MX - but every hexedit should work).

1) Locate your savefiles (%LocalAppdata%\TheBeastInside\Saved\SaveGames)
2) Open the file "SaveGameTypeName.sav" into the hexeditor and search for a string that starts with "DV_C0" - this is the actual chapter with
"DV_C01" being chapter 1, "DV_C02" being chapter 2 and so on.
3) Change the string to the chapter of your liking and save the file.
4) Start the game and click on "continue". The chapter you chose should be the one that's now loading. I am not perfectly sure how the checkpoints are saved so it might be best to reset checkpoints by chosing "restart chapter" from the menu.

Addition: After finishing the game most files will get deleted from your savefile directory and the method described above will only work after you have restarted the game and waited for the first checkpoint to be saved. Also, if you want to test all endings in the final chapter you should quit the game using ALT+F4 the moment "The End" is shown on the screen. This way you can simply reload the game and change your decision.