I don't disagree with you. Werebeasts need to kill to survive, that's a given. The term "monster" can be attributed to the perspective people give men and women who turn to beasts. But thats all it is, a point of view. They're more animal than anything else. Intelligent, however.
How is it a metaphor if you are actually fighting the creature itself? How about the Spirit wolves the werewolf can summon to fight for them? Surely those aren't imaginary beings? It is both mystic and natural. In Bloodmoon, there is an active ability called "Spirit of the Wolf", which heals the player once he sates the beast within by killing a person. Why do you think Hircine has a claim on werewolves? The moment they contract Lycanthropy, they become something else. They are no longer human, they are part animal. As Kodlak said:
"The disease not only seeps into the body, it seeps into the spirit."
Additionally, we have a quote from Daggerfall.
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Or this:
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This alter ego can be their animalistic instinct, their urge to kill and to hunt. Their own bestial mind within them.
Sinding could of turned back into his human form and had been caught bloodied and mad with the girl's pieces stuck in his teeth. It is possible they caught him half naked like he was in the cell. Shirtless with underwear.
What I mean by metaphor is that the spirit wolf hasn't been lurking inside the PC and the others the whole time. They were not two seperate beings in one body. The spirit was the manifestation of the disease trying not to be cured via Hircine and that specific magical ritual. I don't think that, when infected, a beast spirit is also transferred in somehow. Does that make sense?
Also I have negative feelings on the whole summon spirit wolves and Werewolves thing. Werebeast are not supposed to be able to do magic. It would be different and better if the howl was 'calling' flesh and blood wolves and Werewolves to help out. Thankfully this seems to only be a consequence of that totem and not a canon Werebeast ability.
I think of that as more about how intoxicating and addicting the beastblood is. To me, "the call of the blood" came across more of as an urge to transform and sate the beastblood, similar to how smokers get nicotine cravings and recovering alcoholics are frequently tempted to drink again.
It seemed much less a Jekyll and Hyde thing and more of a resisting temptations ordeal.
But wouldn't these temptations be their own . . .erh, alter egos? Or perhaps I am using alter ego's incorrectly

That is what I mean. When Werebeast transform they are still themselves but with a different set of cravings and instincts instead of two minds occupying one body.
I feel like it is implied that they do not have the transformations since for one, they got their form of Lycanthropy through magic with the Witches, rather than being bitten. Also, they make it seem like their form of werewolves are rare since the only other one like them is Arnjborn because Sinding is a regular werewolf. Also, think of it this way, The Companions live in Whiterun, that is the main city of Skyrim, the trading crossroad, so it is supposed to be busy. I doubt they could get away with having to leave Jorrvaskr every full moon to transform because somebody would catch on eventually, plus that is inconvenient for them since they are mercenaries. I believe they are a special strain of L:ycanthropy with no lunar transformations, and yes it was because of Bethesda's poor gamplay design, but they added lore in for it at least.
It is never stated exactly how they recived the curse from the witches. I doubt it was from some sort of spell though that is not impossible. The Glenmoril Witches have a unique relationship with Hircine and so it is not like sorcerors are going around cursing people with Lycanthropy. And if they were why would that method of infection produce an alternate strain? Knowing Hircine and what his experience has been like in previous games it is not unfounded to consider that whatever ritual or test the Companion that began this trend had included an actual Werewolf and thus the source of the disease. There are no stated differences between Arnjborn, Sinding or the Companions nor was any lore changed or added. And I think that the fact that they are mercenaries is very convenient for their situation. They have access to prey that they would be sent to kill anyway. Werebeast are widely thought to be myths and legends and so no one is looking for them, except hunters in the know like the Silver Hand.
I consider an alter ego to be an alternate persona that someone has, either one that was created by the original personality to serve a specific purpose, or a completely separate personality within the same physical body (though this second personality must be native to this body, not an outside entity possessing it). The former is usually seen in superhero stories (for example, Batman is Bruce Wayne's alter ego, Spiderman is Peter Parker's), but can be seen in other media (such as Walter White and Heisenberg in Breaking Bad), while the second is what we see in people with dissociative personality disorder (also known as multiple/split personality disorder), which we see in characters like Dr. Jekyll and Mr. Hyde, Harvey Dent and Two-Face, and most interpretations of Bruce Banner and The Hulk.
The beastform could be considered an alter ego in the truest sense (a personality completely distinct and separate from the original much like Dr. Jekyll and Mr. Hyde) if the afflicted individual had no agency over their actions when transformed, similar to how werewolves have been portrayed in various media. However, in TES, the beastform still has the humanoid personality in it, provided the individual hasn't been overwhelmed by the beastblood.
My thoughts exactly. Werewolves and the other Werebeast are not like the others in various other media. It has seemed to me for a while through playing and reading through several TES games that Werebeast did have control but were complelled to kill because of their Bloodlust and knowledge that they would die otherwise. I think it is similar to the Hulk transformation. They are still themselves but are bombarded with all the new heightened senses, bloodlust and energy.