Not matter to whom? It may matter to those who knew you, and your victim, and will cause a ripple of inconvenience to everyone attached to those lives, so in that sense it will matter to those people to varying levels. Within the grand scheme of things, a generation from now, no, it won't matter. Just an insignificant statistic from the past.
Of course, to you it will matter, you'll;
1. go to prison for a long time. It will hurt.
2. Your social position will be forever damaged, and social position is very coded in to our social monkey brains. That's what having a conscience is all about.
3. After prison our options for good food / safety / breeding will be severely compromised, your life will suck.
So you don't senselessly murder. Even if you could get away with it, you're not going to unless the sum of your anguish outweighs your control over the situation and yourself. Unless you have a brain disorder or some apparent justification, it's just not in your nature. You may feel like murdering, but if you picture yourself standing over a someone's bullet-shattered face, their breath gurgling up through strips of torn flesh and cartilage, painting the floor red as they spasm about in catonic pain... eh, your brain is wired for empathy, that's the meaning of why you can't go there.