You've literally posited nothing to reinforce your position other than he's flirtatious and wants to be a mercenary. What substantive behaviors showed he was devil-may-care, anti-heroic, less naive, and vaguely assholish?
"Assholish" was your word, btw.
then allow me to be as clear as possible:
- mans is a hedgehog with ~5 lines. the primary source of information about zack's overall personality is given to us by characters' insistence that dizzy!cloud is very similar to him, a similarity striking enough (sword notwithstanding) that it catches the attention of zack's ex-girlfriend.
- though the severity of cloud's dickhead syndrome is largely (player) determinant, his constant (and defining) characteristics are his self-assurance (sometimes to the point of being cocky or arrogant), his charisma (especially relative to his real self), and his pride in his title and work (despite personal distaste for his former employer).
- zack was at the highest echelons of the company's [imperial] military force. he wasn't trying to be; he was. given this, he was at best witness and was (much more) likely party to targeted killings, acts of destabilisation, subversion, other special operations and what-have-you. obviously the exact nature of the organisation's activities is pure speculation, but given sephiroth's career (and his celebrity in-universe), we can at least safely conclude they involve quite a bit of bloodshed.
- despite this, at no point does zack express misgivings about his position or his involvement with the company (granted the scope of his role doesn't allow him much opportunity)
until they harm him. from the little we see, i question if he would have defected at all if shinra had an even moderately reasonable response to the nibelheim incident and hadn't handed him down to the local crackhead with the phd from the university of clownery.
- immediately after escaping, his plan is to run to shinra's seat of power (??) and "get rich" plying the same skills he learnt from them as part of the circuit. and whilst i have my doubts that this would include...assassination or something, nothing about his so-far expressed scruples (or lack thereof) and his (apparent) lack of reflection encourage me to believe all these jobs would toe the line of moral discretion, particularly given a city like midgar. for his own part, cloud did become a mercenary. and immediately fell into it with a gang of terrorists.
the confluence of all of these things -- with the added cliche of a somewhat superficial approach to romantic entanglements that the woman/his partner is far more invested in -- is what suggests to me the (very 90's) archetype of the fast and loose anti-hero who is mostly heroic in name only, and represents a certain kind of masculine ideal. these kind of characters are usually still meant to be sympathetic regardless, and are/were humanised by their humour/affability/loyalty to individuals/a team/an ideal/whatever. there is also probably a straight line that you could trace from this type of hero to wartime/post-war/pro-state literature and propaganda, but i realised like two sentences into this post that i don't give that much of a shit about all this. so i will not.
i am not saying that zack ran around town scalping old folks with butter knives. but someone who is charismatic, given to impulsivity, has a talent for violence and enacting it (that he doesn't try to interrogate or abandon), and doesn't really think too deeply about his actions (even and especially when they are consistently heinous with heinous consequences) unless they negatively affect him...all these combine into a megazord of traits that i, personally, would at least describe as "vaguely assholish." perhaps you would use a different term. but for me, regardless of their interpersonal skills, that is not the type of person i would enjoy being around.
He was a cog in the war machine who was good at what he did and by the time he planned to quit and move on, life twisted and it was too late.
he was a frontline super soldier who climbed the ranks through efficient and lethal executions of shinra's will (and like...literal executions also). not a financial management officer hunched over a desk on floor 69 (kek). his precise duties would involve a large amount of theorisation, but unless you take his characterisation in the compilation and retroactively use it to interpret what we see of him in ffvii [and even his development in cc revolves mostly around the effect shinra has on him and the people close to him, not necessarily the effect it had on his victims], the idea that zack existed as some all-loving hero with the heart of superman is not convincing to me. the original game gives me zero reason to believe that at all...even if he truly did think what he was doing was moral.
for that matter, the og doesn't give me cause to believe it's even possible for a character like that to exist (particularly within zack's circumstances) in it's world. it seems to (thematically) go out of its way to discourage that. even with literal saviours like aerith.
edit:
Just out of interest, how do we know that it wasn't Wutai who started the war with Shinra? Just because Shinra is an amoral power-hungry corporation, it doesn't necessarily follow that they must have been the ones who started the war. We don't really know much about this war except that SOLDIER fought in it and Wutai suffered a humiliating defeat. For all we know, pre-war Wutai could have been as aggressive as pre-war Japan in a Pacific region dominated by the USA and Britain.
Yah, plus the fact Wutai stood up to Shinra for 10 years or so suggests they weren't some peaceful farming nation...
unless i'm just dumb (very possible) and have shit memory (i do), the war was instigated by shinra when wutai refused to allow a reactor to be built on their soil (and by extension, submit to economic domination). and a nation doesn't have to be completely helpless or pacifistic for an imperial campaign launched against them (and attempts to undermine their government) to be illegal/immoral.
in any case, i honestly can't remember if all that was compilation nonsense or not, but i think i vaguely remember the suggestion it was defensive regardeless.
The original game never established that he personally had a role in the Wutai War.
Did nobody invent a more proper term for his case?