People think that, and sometimes they can do it, but without feeling anything? With no regret? It’s far less common than you think. Even in cases where killing is totally justified, people will find they hesitate at the last second, because it’s NOT a rational part of your brain that you’re dealing with, it’s a deep-seated instinct that has to be overcome, and even if you manage to do it, guilt is liable to follow no matter what.
Again, if someone can kill someone and feel absolutely nothing, then they’ve either been extensively desensitized (which military and police have detailed procedures for training – and it still doesn’t work 100%, lots of them wind up wracked with guilt for the rest of their lives) or those instincts just don’t function in them because they are sociopaths. Either way, they are not normal. Something happened to make them that way, or they were born different.