Your likely looking for the app “Puzzles” ob Android or one of its desktop versions, I am not entirely sure if they are actually open source but there are at least some people’s contributions and it not only has “Killer” sudoku but a whole bunch of additional options (as well as tons of other awesome puzzle/logic games)!
Simon Tathams Puzzles
PlayStore https://play.google.com/store/apps/details?id=name.boyle.chris.sgtpuzzles
Website: https://www.chiark.greenend.org.uk/~sgtatham/puzzles/
Have fun solving them! :D