When you don't fully support gun rights, that's a deal breaker, no matter who you are. From Stripes:
A group of former senior military officials and veterans, including Gens. David Petraeus and Stanley McChrystal, have launched a veteran-focused gun law initiative.
The initiative, Veterans Coalition for Common Sense, is spearheaded by Capt. Mark Kelly, a former Navy combat pilot and astronaut whose wife, Rep. Gabby Giffords, D-Ariz., was the target of an assassination attempt in 2011.
The couple jointly formed Americans for Responsible Solutions, a gun control advocacy group in the wake of the Newtown, Connecticut shootings in 2012 that left 27 dead, most of them children.
The new gun control initiative, which is a rip off of another veterans' advocacy group, is from Mark Kelly and Gabby Giffords's other gun control group, Americans for Responsible Solutions.
Petraeus is currently being considered for Secretary of State. Why is his stance, a potential Secretary of State's stance on gun control important? Perhaps you've heard of the UN arms treaty?
Gun control lobbyists are enthusiastic about exploiting veteran suicides as a way to enact further gun restrictions. They think that ironically positioning veterans at the forefront of a rights grab initiative insulates them from civilian criticism. These are the same people who turned the other way after horrible VA mismanagement under the current administration which saw few people fired and others receive bonuses. The wait times still exist. The problems still exist. If this was a serious effort at helping veterans, they would have formed a VA accountability group or pressured the administration to overhaul the VA, which President Obama has been reluctant to do.
Because of this alone (to say nothing of this), Petraeus is a wholly unacceptable candidate for Secretary of State.