I hate flies. Hate 'em. Despise them.
Okay, let me qualify that by adding "in the house" to the above.
Seriously, a long time ago, back in my Army days, a couple of flies got into my apartment and, like horny little flies tend to do, had a mating session. A few days later (by which I remained totally unaware of the fly porn going on in my abode) I awoke to a pretty nasty odor. The odor was so strong it, in fact, is what woke me up in the first place. Upon investigation (which took, oh, three seconds), I found a trail of maggots. A trail of hundreds of maggots. And I quickly learned that Raid and other fly-killing chemicals do not kill maggots.
So I swept, stomped, and spewed until all of the maggots I could find were in the dumpster in the parking lot.
A couple of days after that I awoke to the unmistakable sound of hundreds of flies buzzing. They were mainly on the sliding glass door, trying to get out into the backyard. Long-story short... out came the Raid, down went the flies. And I quickly learned that I hate the smell of Raid. I need not mention the health risk pesticides under pressure post towards humans.
So, using what I'd learned about other insects, I switched from Raid to degreasers. And natural degreasers, at that.
What I'm trying to say here is that, while Raid might be the best option for killing hundreds of flies at the same time, Clorox's "Green Works" is the best thing I've found to kill flies in small numbers. It's safer (for you and the environment), smells better, and allows you to spot-clean while killing pesky little winged insects at the same time. And since the chemical simply sticks to the fly's wings, making it too heavy to zip away, it gives "I love all life" environmentalist-types the option of picking the fly up and putting it outside.
Me? I just crush 'em. Because I hate flies.