Should I kill it?

For the past couple of days every time I’ve gone into my bathroom I’ve noticed a housefly wandering around on the window. It meanders slowly up and down the glass, just minding its own business and never buzzing around or being annoying. So I’ve left it alone.

A few minutes ago I looked a little closer and I noticed that one of its wings is deformed. I tapped the glass next to it and it freaked out and fell onto the sill below, then got up, got back onto the glass, and continued to walk around (though rather frantically now). So apparently, it can’t fly with its mangled wing.

Should I put the little (actually, it’s pretty friggin’ big) flightless fly out of its misery? Or should I continue to let it journey up and down the window peacefully until it reaches the end of its lifespan naturally and dies of old age? I mean, can it find food there on the window or will it starve to death? And how did it get there, anyway? Actually, how did it even manage to grow to this stage, really, with such a deformity? Isn’t nature usually pretty merciless when it comes to things like that?

[Mods, if I may, I’d like to ask that silly answers not be deleted, even if they’re given first. I’m not even sure how seriously I take this question, even though I honestly want to know what others would do and what they think I should do.]