I think "Murder mode" is stealth mode, where you sneak up behind your enemies and kill them.
We already know how Halo Reach ends, so I assume the game plot involves holding off the covenant while "valuable assets" get evacuated before Reach falls.
Yet, today, America's leaders are reenacting every folly that brought these great powers [Russia, Germany, and Japan] to ruin -- from arrogance and hubris, to assertions of global hegemony, to imperial overstretch, to trumpeting new 'crusades,' to handing out war guarantees to regions and countries where Americans have never fought before. We are piling up the kind of commitments that produced the greatest disasters of the twentieth century.
— Pat Buchanan – A Republic, Not an Empire