By "death magic" I assume you mean Black Arrow as it was designed for MM in Legion where it summoned an undead boar to fight for you?
First off, that design was part of MM, not RSV.
Secondly, Black Arrow as designed for RSV never had anything to do with "death magic" and "necromancy". It dealt Shadow-damage, that's all.
You can find many poisons and other toxins in this game, and in the RPG, which have nothing to do with actual death, while still focusing on Shadow-damage.
And as for your argument about it being an inconcistent mess, having a spec which works with poisons, animal venom and explosives etc. This is just a case of you not understanding what they all amount to. And why/how they DO make for a coherent theme.
Sure, unlike Mages where one spec focuses almost entirely on a single school of magic, RSV did not only focus on one of: physical/nature/fire/shadow damage etc. It technically only focused on Arcane and Shadow, as actual magical damage.
But the above is not where you should look in terms of determining coherency. But instead, like you said, RSV was a spec that focused on different types of augmented shots/projectiles.
No matter if it was an explosive or a poison or animal venom, or magic, they were all designed with the same coherent theme in mind. Augmented shots/projectiles.
And it all makes perfect sense as to why you would, as a munitions expert, want to be able to utilize many types of augmentations/enhancements.
It wasn't "Marksman but with special ammo".
Marksman focused/focuses on the theme of a sharpshooter/master archer. One who is skilled at handling their ranged weapon. The impact and the point of impact is what matters. RSV never focused on that theme at all. It specifically focused on enhanced/augmented shots because it did not intend for you to focus on having perfect aim.
And, as for that second part, it sounds as if you're currently missing the basics of what specializations are about. Yes, ofc you can design MM to also be about augmented shots. The point with specializations isn't that each respective theme simply cannot function with one another but instead, it's about giving us/presenting us with multiple distinguished choices.
Specializations are there for US to choose for ourselves as to which part(s) of the class we want to focus on. Again, yes, ofc a MM hunter could focus on using special ammo. The point with specializations is that you're choosing not to. You as a player choose to opt for a particular fantasy.