Malus Necessarium (or Malum Necessarium) is a policy applied to actions, prescribing the use of as little evil (as in “suffering”, “malice” or “evil”) as possible to achieve a means. It is a necessary condition stemming from utilitarianism and literally translates from Latin into “Necessary Evil”. The wording is derived from the Hitman games, which uses a similarly named term to describe it's way it wants the player to play the game, by assassinating the targets while causing the least amount of collateral necessary. The game discourages and punishes unnecessary kills, injuries etc and so on.

“Malus Necessarium” is a theorem derived from the axiom of utilitarianism: Create as much good as possible, prevent or avoid as much harm as possible.
Therefore, in planning our actions we are to find ways that cause as little harm as we can. Utilitarianism may mean as much as “the ends justify the means”, but really it means “the ends justify the means which cause at least damage and the most benefit”. Tradeoffs between causing more damage but also attaining more benefit, or causing less damage but also attaining less benefit, etc., are up for debate. Until then, unnecessary harm must be avoided if there are ways to reach the same end, ie. cause the same benefit, while incurring less damage.

Of course, one can always forfeit their own claim on receiving benefit/not being subjected to damage, but this is either completely voluntary or on the basis of a moral contract consented to by all involved parties. Also, this only affects yourself. Unless you have consent by everyone else you want to involve, voluntarily putting yourself at a disadvantage must only ever affect yourself and never involve other people around you.