Killing intent was a weapon and it became even more powerful when it was used correctly. You could just emanate it from your body, and it would act as a weight towards your opponent making it seem like the area around them was weighing them down. If it was used on weaker individuals that lacked will and mental fortitude, then it could kill on its own. As Havok had done when he first arrived in the city against a random man that had flashed killing intent towards him. When he released his killing intent on that young man, it short circuited his brain due to fear, killing him instantly.
Although that young man had killing intent, he had most likely never faced any from someone that had it at the level that Havok did. Comparing their killing intents was equivalent to comparing the flame from a lighter and the sun.
