How can one argue that a wacko criminal that kills a bunch of people with his AR-15 should be charged with the crime,
The act clearly was perpetrated by an individual, and government has an obligation to identify that individual.
but other AR-15 owners should not (individual responsibility theory)
Those owners are clearly uninvolved. That they happen to have the same kind of inanimate object as the criminal is irrelevant. There is no discernable connection between the criminal and other uninvolved owners.
and then argue that we should invade an entire country for the actions of a few of that countrie's citizens (collective responsibility theory)?
Expediency for survival. When some individuals are attacking you, you're gonna suffer grave harm unless you "take 'em out" - which may mean "taking out" some uninvolved parties in the process; acting to minimize harm to innocents may greatly increase your chances of prompt death. Morally, one is obligated to minimize harm to others ... but when that act unduly puts oneself at grave risk, others do not have the right to criticize one's subsequend prioritization.
If someone is actively trying to kill you at a crowded shopping mall, a proper jury will not convict you for doing an appropriate job of shooting to stop that assailant - even if others get harmed in the process. In a war, soldiers have a duty to do everything reasonably neccessary to terminate their opponents - even if others get killed. When "total war" was introduced in WWII by the aggressors, the defenders were justified in firebombing whole cities - tragic, yes, but the only reasonable option to end hostilities with, relatively speaking, minimal losses. It's called "collateral damage".
One certainly should refrain from unnecessary violence, limiting dished-out harm to those individuals who will only stop, or be properly punished, by appropriate force. The AR15 owners in your example had nothing to do with the crime committed; acting against them involves seeking them out and making them involved when they were absolutely not involved. In war, law has broken down and the only imperative is to stop the aggressor through overwhelming
but not excessive force; if my wartime opponent is trying to kill me _right_now_ and the most appropriately expedient solution is to level the building he's hiding in (say, sniper on the top floor, and I have no suitable cover), fine - but it is NOT fine to deliberately act to level the building next to it if doing so is reasonably avoidable and the desired outcome can be achieved without doing so.
The confusion in your mind likely comes from a misunderstanding of accuracy under fire (whatever the weapons involved), and of the timespans involved. "Detatched reflection is not required in the presence of an upraised knife." That a third party is harmed is NOT BY CHOICE, but by statistical likelyhoods while pursuing a high-priority goal.
Yes, there are innocents present in wars. We have no interest in harming them, and - when possible - will act to not harm them. However, the priority in war (like any form of self-defense) is STOP YOUR OPPONENT FROM HARMING YOU. Sometimes the choice becomes: risk harm to innocents, or risk harm to yourself - one or the other, no third option. Much as we try to focus our efforts on the individual in question, sometimes we can't avoid others and the issue becomes a straightforward matter of priorities: you, him, or bystanders.
The problem with, in your example, disarming AR15 owners is that they are not part of the situation. Gun-grabbers erroniously think they are, but are uninformed/ignorant/deluded/bigoted. Shooting at someone shooting at you is reasonable; risking the unintentional shooting of those behind him is prioritization; turning around and shooting the clearly & completely uninvolved person behind you is wrong.
As for invading a whole country: there is a system in place which, for reasons of defense, must be stopped, controlled, and/or replaced. Government is pervasive throughout a nation. Taking out Saddam did not completely remove the threat involved, the entire control network had to be shut down (much of it by force), removed, and a new system put in place (lest we leave anarchy behind, which is immoral); not invading, removing the opposition, and establishing a new government was not a moral option. Those resisting must be dealt with, and sometimes at the expense of innocent bystanders, lest the final new system collapse into greater death and despair. The above applies in light of the appropriate central goal; removing Saddam from power is far harder done than said.
Get it?