This concept has been done to death, but it's still a good one.
As it stands now, it's way too simple to give me any excitement, but an actual "upgrade your death ray" mechanic would make this enjoyable.
I know the aim was to make a simple game, but that just doesn't cut it anymore. Instead of beating up everything, it would've been much more interesting to barter with people for specific parts(like tesla capacitors, laser targeting systems or something funnier, you can go crazy with names). Obviously you would need to get these developed parts from the better scientists, explaining how is Vile the worst. With interesting characters, funny dialogs, death ray developement, and a little logic this could turn out good.
By the way, I disagree with the other comments, beating people is easy, you just need to get the hang of it(and you get that after the first fight you have..). You can keep it as a side mechanic, but it shouldn't be the focus.