Most people in western societies don't understand the concept of karma very well, or at least they don't articulate it in a way that's consistent with its traditional spiritual meaning. Naturally, as a bit of a pedant, it seems important to me to correct this misconception. Karma is something that happens on the scale of multiple lifetimes, like evolution, not shorthand for "payback is a bitch". Of course, that's something a bit vague in a culture like ours where "forever" is usually used to represent "until I die" as opposed to, well, forever. Still, it's an important distinction.
But, like any concept of faith which transcends an individual's lifetime, some people use the "well I'll be dead then" argument to justify being unkind. I like to see it as justification for the opposite behavior; There's something quite appealing about the sense that progress is based on the sum total of your decisions and actions. Leaves a little more room for making mistakes and meandering around a bit while figuring out how to live one's life.
Usually when I articulate the actual meaning of karma to people, which is usually in a context somewhat akin to correcting their grammar, I immediately get the question, "Well, if it only happens after you die, what do you think of Instant Karma?" After hastening to point out that karma doesn't happen after you die, but rather as you progress to your next life, I usually explain that, to me at least, something like John Lennon's work is an artistic expression of a concept, and one that I find pretty appealing. Instant karma doesn't exist, of course, in Hindu or Buddhist tradition. But it's kind of like trying to Do the Evolution, a nice way to try to get control of something that's bigger than just your own life, or lifetime.