Hasan told family members that he wanted to be discharged from the Army. How hard is it to get discharged? Very hard.

Except that officers can resign their commission. The only thing that might prevent that is if they have incurred a service commitment for, say, the government paying for med school. Then the gov wants to earn it's money back (your tax dollars). It may be Hasan had a service commitment.

Now an enlisted person getting discharged before their contract is up is a bit harder.

Hasan could have avoided deployment by shooting himself in the foot... but he'd probably be discharged and have to pay back for med school.