First of all, the killer instinct is basically the unquenchable will to win at all costs - a natural competitiveness which comes out in every meet and even in training. Every swimmer has this instinct to some degree, but some more than others. This is NOT, of course, to say that the swimmer must hate their competitors - I remember one coach trying to encourage this attitude in a gentle swimmer who was simply 'incapable' of hating anyone - but the killer instinct DOES prevent a swimmer from slowing down for their competitors, even if they DO feel sorry for them (and believe it or not, this is quite common).
Feeling compassion for other swimmers is not a bad thing - in fact it shows the markings of a highly decent person, however this attitude should NOT change the way you swim in the pool. Feeling compassion is one thing, but slowing down is definitely another!
However (believe it or not) there are many swimmers out there who purely and simply swim because they LOVE it - not because they have to win all the time. This is a wonderful attitude which creates swimmers who will probably swim all their lives, however sometimes these swimmers can find their results suffer occasionally because they discover they do not possess that grim determination to 'finish a race off' when they should. This attitude may seem unbelievable to highly-competitive swimmers, who probably never experience this problem! (Let me also mention that it is, of course, also possible to be at the other end of the scale - that is, being a little too "over-competitive", and this attitude can sometimes produce great amounts of stress, and sometimes even 'undesirable tactics' such as intimidating competitors before races, etc.
But let's get back to the subject of the killer instinct. How does a swimmer increase their motivation or 'competitiveness' in order to achieve greater results? Well, there are a few things you can do - the first is to write down some goals you would like to achieve, which will create a direction you can aim for. This is your starting point.
Secondly you can mentally 'model' your attitude on that of a highly successful swimmer you admire, copying their attitudes and behavior until they become your own. This is very powerful and most successful people have modeled themselves on others at some point in time. And thirdly, you can visualize or imagine your goals being achieved, by regularly allowing 'successful' images and thoughts of your swimming to wash through your powerful subconscious mind. This will create a 'blueprint' for success and increase your motivation to achieve your goals.
So don't worry, nice people DON'T have to come last! You can still achieve your desires, and be a nice person too - and their are many champions out there who prove this. So if you weren't born with a true 'killer instinct', don't worry, there are still ways for you to come out on top.
The Mind controls the body, and the mind is unlimited. The best of success,