So you argument is that it's fine to not care for insects because there are a lot of them, and because their lives are short and brutal.
I would say the opposite. Animals whose lives are rife with pain and misfortune can use all the help anyone is willing to offer. Granted, I will not seek out every fly and try to "save" it. But if I come across one I can help, I will do so. I think you're making the argument "either you HAVE to care about every fly or you CANNOT care about any flies." Perhaps that logic works for you, but I think it's a fallacy. I can care about flies without needing to care about the well-being of every fly. I also adhere to non-interference. If a fly is caught in a spider web, I will leave it. To free the fly is to starve the spider. Some things we just don't have control over. But I will do my best personally to not harm other animals.
I said this in another comment, but I'll repeat:
> I will show kindness to animals, but only to the point where it doesn't completely impede my life or my ability to find happiness.
In other words, it doesn't impede life or my happiness to catch a fly and bring it outside. Therefor it falls under the "show kindness to animals" category.
(I should add an addendum to that: any animal that is attacking me physically is subject to death (horse flies, mosquitoes, dogs, bears, etc)).
Given all that, I do not judge people who don't care about insects. I get it, completely. But that doesn't mean I won't stop challenging people's viewpoint on why it's ok to kill some animals and not others.