There is a difference between a bar fight and killing someone with intent. A bar fight is a fight for dominance, like you see in animals, the outcome is rarely fatal as the point is just to establish who is stronger and who should submit.

We have an aversion to killing that goes further than the fear of consequences. It can be seen in the military, where soldiers naturally don't want to kill their enemies, even when they have incentives to do so. To be effective, soldiers have to be desensitized to killing through their training. Enemies are dehumanized, they train on human shaped targets, etc... And even with all that training, after a few years of active service, many start questioning their life choices. It is common for military pilots, who enlist for the love of flying, until they realize what they are really doing, i.e. killing people. When that happens, it is time to retire to a noncombatant job.

Punishing crime is not useless, but I think saying that consequences are the cause of aversion for killing is backwards. We have a natural aversion for killing, especially when we are in a prosperous situation like we (the first world) are now in. And that's why we take murder so seriously.

And speaking of murder, our natural aversion for killing shows when we see how we treat the death penalty nowadays. The death penalty is (generally) for the worst people humanity has to offer, their killing have been approved by the highest authority, and there is still opposition. We even have rituals to offload the responsibility of killing from the executioners. For example by having a random person in a firing squad fire a blank round.