It's not sadism or being a psychopath. It's just habit or boredom when I'm sitting a picnic with my girlfriend and I see them crawling on to my sneakers (annoying). Or when I'm playing soccer with my friends and I slowly stomp an anthill with one cleat to see how many I can crush at once. I don't do it for any particular reason... I just get a kick out of it. It's like asking someone why they kick a rock when they can just as easily not.
Why should I care about the lives of little bugs that just crawl around and move stuff? As far as I care, I'm a higher and much huger life form, so my temporary amusement is more important to me than their tiny little lives. If the last thing the bastards get to see over their heads are the bottom of my giant, stinking feet about to crunch their puny little selves into jelly because I think it's funny, then tough luck for them. It's survival of the biggest.
Disagree?