One needs to keep in mind the objectives of the plotters (of all the 15 attempts). It was not that Hitler was running the war badly-it was that he started a war that any sane strategist could see, would only result in Germany's defeat.
The plan of every plot was to kill/try Hitler, and immediately open negotiations with the Allies. Such a move would've thrown the "unconditional surrender" statement-which wasn't strategy it was an offhand reply to a reporter's question btw-into chaos.
Certainly an assassination, would've started the dismantling of the Nazi regime-a stated Allied war aim.
Such a surrender would've kept the Soviets out of eastern Europe, and saved literally millions of lives.
One also cannot discount the personal motives of those who wished to kill him, and there were A LOT more German officers who wanted him dead than Allied propaganda would like one to believe: the idea that not all Germans were supporters of Hitler, and had the strength of conscience to do something about it.
EDIT: For those who insist about "Barbarossa" destroying the Third Reich, I point you to do documented evidence in the Russian, Swedish, Bulgarian and German archives that Stalin tried to surrender to Germany in late 1941: ceding essentially the same territory Lenin did in 1918. He also tried to cut a deal with the Finns, returning territory captured during the Winter War if they would drop out of the war (only fears of German occupations kept the Finns from accepting this offer). Stalin tried AGAIN in spring 1942 when it looked like the Germans were about to cut the USSR in two at the Volga.