I also made a similar game once, where you try to not kill your past selves. The fun thing about this kind of game is that the gameplay really changes depending on your past actions. I've managed to find myself a space in the leaderboard by playing really calm and it felt like a supermarket queue for killing. But it's also possible to create an action war zone full of stray bullets. The game is really polished and the style is coherent and cool. I really like the blackhole effects. 