No, mass murderers don't need guns but if you compare mass stabbings to mass shootings, the shootings generally have many more deaths. Guns also make it fundamentally easier than many other alternative methods. There is data that suicide is more likely with a gun due to the fact that gun suicides are much more successful than other methods which offer an option to change one's mind and reverse their action.
Let me back up a little and try to illustrate the pointlessness of debating whether banning guns would save more lives. If they did a bunch of studies and could conclusively show that banning all guns would significantly reduce deaths (both homicides and suicides) would you concede defeat? If they could prove that homicide rates would fall greatly, would you be willing to give up your guns? If your answer was no then it was pointless to argue whether legal guns cause more deaths or not because you aren't going to give up your guns either way. Guns serve a number of important roles so even if banning guns would conclusively result in a net number of lives saved I wouldn't give them up. Therefore I don't choose to place my eggs in that basket because my justification for owning a firearm doesn't rest on trying to prove guns save more lives than they cost.
Banning cigarettes and alcohol would definitely result in a net number of lives saved but no one is willing to give up alcohol so the debate about how many lives would be saved ends up being moot.
Defeat to whom?
The anti-gunners who want to pretend they can take away all legal guns, and that somehow magically all illegal guns will no longer be available?
The politicians who paint every problem as a simplistic subset of causes that never fix anything?
The only "defeat" I would concede is the loss of our second amendment, and the loss of freedoms to a government that already displays a desire to trample on individual rights time after time.
That's a defeat that other countries have experienced, only to wish they had their guns back when everything went to hell in a hand basket as the government leaders raided the treasury and private industry, then left the public to fend for itself in the face of armed police and military.
There's no Constitutionally protected right to vices: booze, tobacco, etc. Funny how the ATF regulates firearms in addition to alcohol and tobacco, even though one of them is supposed to be left alone without infringement. Anyway, arguing that banning a substance to save lives can be a valid discussion, since there is no civil right being violated. Guns are on the opposite end of that spectrum.
You like to create hypothetical straw arguments. "If" a study says fewer guns in the hands of law abiding gun owners saves more lives than it costs, and "if" the study can be proven independently across a broad variety of demographics, social environments, etc., then I'd be able to view the data and make a judgment. Asking if I'd make that choice before any details are offered is a red herring, meant to paint me as uncompromising. That's a leftist tactic, meant to prove your case without having to prove anything.
Ever see the movie "The Village?" The residents all had some tragedy in their lives that they blamed on society. So, they all moved into an isolated village to escape the forces of violence and bad decision making that lead to their life altering events.
The result was a murder -- something many residents joined the village to ensure would never happened again.
Humans love to think they can control their destinies by forcing others to live by rules that limit freedom of choice.