If you want a diamond necklace that you can’t afford, it is necessary to steal it in order to have it.
It is not justified to steal it simply because it was necessary to meet your goals.
You are implicitly assuming that the necessity of self-preservation equates justification on the premise that self-preservation is a just result.
I don’t agree.
If two soldiers are fighting for their lives against each other, it may be necessary for each to survive to kill the other.
But the family of the one that dies may not see their loved one’s death as justified even if the family of the one that survived sees it that way.
Your self-preservation is worthless to me, and thus justifies nothing. My own self-preservation is literally worth everything to me - and yet if still does not justify my taking everything from you, even if I deem it necessary to achieve my own desires and goals, any more than my desire for a necklace I cannot afford justifies its theft.
There is a distinction between things like stealing bread to save a life where a necessary action is justified by the good that comes out of it and stealing bread to throw away in order to achieve a thrill. Both are necessary to their goals, but one has a goal that justifies the necessary action while the other does not.
I’m saying that there is no goal or good in existence that justifies the inherit evil of mass violence, even if there are a myriad of ways in which mass violence might be necessary to one’s goals, with those ranging from ethnic cleansing to fighting tyranny.