While that killer instinct isn't something that you can learn from an article (so I'm wasting my time writing this one theoretically...), and you're not going to go around starting fights so you have the upper hand, there are thankfully several ways you can use your own psychological techniques to get the best of your attacker and you can develop at least a more pragmatic attitude to confrontation.
Stop Pulling PunchesIt's a fact that everyone naturally pulls there punches. Even when you think you are hitting someone with your full force there is a mental barrier that is preventing you from using 100%. This is actually the reason that insane people often seem inhumanly strong - because their mental barrier is missing. It's also the reason that you hear stories of children getting trapped underneath cars only for their mothers to find the strength to lift it off of them. In a fight this mental barrier becomes even more apparent. Most of us are averse to violence and will start to feel very uncomfortable when someone starts threatening us. A lot of things run through our minds - we don't want to hurt them more than necessary, we don't want to be 'unfair or unreasonable' and further more we don't want to get hurt ourselves. The attacker however has no such qualms. For this reason a lot of us don't only pull our punches - but become frozen and fail to punch at all - and it's because you're worried about them ridiculously enough.
First of all then, forget about fairness straight off. If they're threatening you then you are perfectly within your rights to spark them out. They deserve it. If you are in any way nervous about hitting them they will sense it and make their move more confidently. You must be willing to respond in kind if called for. The best and fairest thing to do in this situation would simply be to punch them straight in the face they minute they invade your personal space or do that swaggering walk. However most of us aren't willing to do that. Thus we need to be a little more tactical than our thug friends. Surely you can outthink a thug. More on that later, however if you can switch off your common decency a little it will help you out.
Next you need to try and eradicate any fears of getting hurt yourself as that can often cause you to hold back; if it's between getting stabbed and damaging your knuckles I know what I'd choose. One way I overcome this is to pretend I'm the Terminator. In my mind I just imagine being invincible, walking unstoppably towards my target and letting blows bounce off me like rubber. Another psychological technique that's actually used in sports as well is to imagine you have all of your strengths and all of the strengths of your opponent and it's amazing how effective this little bit of self talk can be. Similarly you can pretend you're fighting one of your friends (and practice doing so) so it won't seem like such a big deal. Right so so far we've got the following advice: get over your urge to not cause bleeding (yours or theirs) and use alternative attacks to just punching. Another great one is to grab the head and twist it to the left or right violently which forces anyone to drop to the ground.