I would like to argue that people being rational can murder. You could argue that if somebody with the capability to kill knew that somebody had plans to/was causing harm (though not illegal) and also knew that the effort needed to stop them was rather significant and/or require too much time, committing murder would be a rational choice to stop harm. But this is going to extremes isn't it?
Otherwise, you do make a pretty good point. Gun control isn't about stopping those with enough commitment/desire to do evil, it's about stopping the impulsive. The other question we can ask is how far do we want to go towards stopping them vs inconveniencing law-abiding citizens vs what else can we do to solve the issue. It could be argued that what seems to be little effort has been made towards solving the issue with impulsive people without inconveniencing impulsive people through mental healthcare services, while there seems to be a rather big backing towards inconveniencing law-abiding citizens for the sake of what seems to be a few impulsive people. Not to mention that improving mental healthcare would also help on a wide range of issues such as depression, anxiety, stress, various disorders.
Sadly, I do not possess facts about how much is the budget for mental health care, and the whole issue of stopping impulsive people vs inconveniencing law-abiding citizens is entirely subjective and completely pointless to argue (although most can be reasonable and accept background and medical checks to avoid certain cases of violence with guns).