"I just will feel guilty about screwing over another startup"
I like your confidence. Just by copying their idea, you think you will be able to screw their startup? Think again. It may not be as easy it might appear to you.
But again, should you copy the idea or do it better ? Sure, hell yea. Why not ? Most ideas are anyway a derivative of some other idea.
Stop worrying about this. Go build your stuff and ship it.