create a number variable to track your kills, so basically for every enemy killed add 1 to "Kills" number variable, you can use the same logic to track your deaths, then if you want to display this in the game create a text object and set it to display the "Kills" number variable every tick, now it will display your kills where ever you place the kill tracking text object in your game.
I'm not too well knowledgeable on saving data in Construct, but from what I understand you can use Local Storage object to save data from game to game, you can read more about it here;
scirra.com/tutorials/526/how-to-make-savegames