I feel the same way the "hurr durr ant killing asshole" crowd feels about it, but let me explain why in a way that might bring light to my point-of-view:
Those ants, like you and I, are also living beings. While some living things might not think on our level, feel as much pain as we do, or accomplish tasks as significant as us, it's an undeniable fact that they are living, breathing beings. When you kill one, you're doing.. well.. Exactly what it sounds like, lol. You're killing them. That's ending the life of another being. It might not seem as "significant" because it's an ant, but; that ant was born and has lived a life, just as you and I. It might be a very different life, but a life nonetheless and you're denying it the basic right to live when you kill it.
I really think it's just a point-of-view (one that I think is correct, in the scheme of being a pacifist / having good spirituality / whatever else positive you may apply to this ). A lot of people think on scientific levels or put humans above other creatures in terms of importance.. I think that's the wrong way to think about it, the answer is pretty simple. Of course, anyone can and will argue against my point, perhaps saying "we wouldn't get to where we are today if we hadn't caused the deaths of other beings in the process" and.. to that, I can attest to that statement being true, but I still hold mine to be true as well and I pledge not to kill if I don't have to (of course, accidental stepping-on-bugs is a tough one to avoid at all times, so.. can't win everything. ^^;)