We never saw Capt. Tracey in action, only the insane remnant of the man that he was. Why assume he was some kind of jerk before the incident? He did reach the prestigious position of starship captain. Is it so hard to believe that he was a different man before his crew was killed off?
Maybe not hard
to believe, but unlikely.
Anyone who would stoop to the levels that Tracey did was probably not all there to begin with. If he so readily cast aside his morals and ethics, he must logically never have had much of them to start with.
Remember, just because Tracey may have been a good Captain doesn't mean he was a good person
Decker, in a way, gets a free pass, because he suffered a trauma. Something made
him do what he did, and that something was the planetkiller. Tracey has no excuse - he willingly and carefully decided to violate the Prime Directive. He suffered no trauma, no wrenching event that caused him to go cuckoos.