(Likely) Very Common Spider ID Help (Michigan)

Hello, everyone! For some reason I'm having a hard time finding what spider this is, even though I see them all the time. I know they're not poisonous, but we've got a few in the house, and they're just DECIMATING the other spiders. In the picture you can see it getting after another spider, and there are five or six spider bodies on the ground below this one, also dead.

I just want to know what this guy is and maybe why it's so aggressive. We find piles of spider bodies around our house regularly.

I know the picture sucks, like, super bad -- my camera sucks. It's one of those types of spiders with the fat, bulbous rear end, and you can make out the wavy brown pattern on it (hopefully).

http://i.imgur.com/2RzgmtY.jpg