HotS kits are amalgamations of sources regarding the particular hero, and not just an instance of one thing. Zul’jin in HoTs is as much a ‘regular unit’ reference (like Lunara) as he is a reference to himself.
Zul’jin is an axe-thrower from warcraft 2, but those units didn’t really have much for them ‘ability’ wise for reference, so he uses warcraft 3 trolls for his kit.
Troll berserkers could attack 50% faster, but took 50% more damage; Shadow Hunters have a heroic that makes other units invulnerable;
Taz’dingo is a troll battle cry that essential means “stand and fight”, so him having a heroic that prevents him from dying, so he can Stand and Fight, is thematically fitting and somewhat suits the ‘lore’ of Zul’jin being willing to fight to the last.
The %hp gimmick sets him apart from other aa heroes, arguably references that zul’jin became more ‘powerful’ after he lost his eye and arm, but otherwise stands as a form of his outstanding hatred for his enemies.
The main thing for zul’jin is that being an axe thrower was unique to his time during the second war as notable trolls in later generations had other character classes, or rather, used different weapons. Despite having a raid fight that imbued him with various animal spirits, that sort of kit reference could be used on other trolls instead.
So the berserker/axe trope is fairly unique to him, and a kit of shifting hp works well in tandem with troll regeneration, so for the most part, his kit is about as ‘lore’ friendly as he’ll probably get.
Recklessness kicks in at below 75/50% hp for boosting attack and spell power. I’d arguably say it’s better than his stagnant Headhunter talent as he gets the benefits right away, and it synergizes well with his own Amani Rage ability for getting him to a lower threshold for snapshot damage.
Trait/aa is for risky low hp damage, W is for mid-hp mage-gin builds for poke and guillotine snipes, Q build is for safe max-hp damage and bolsters his damage against any marked target, making it his best talent for pve aspects.
I generally consider his design fine, his Q talents underrated, but also the most likely source of ways to ‘fix’ his appeal.
Zul’jin’s kit is less about him taking damage, and more about him giving into his rage. And since he pretty much ended up declaring war on everybody, that’s a suitable gimmick for him.