Well, I simply disagree on there being any purpose to Shadow's death. I don't see it. If a memory of a single girl means more to him than 6 billion lives, and depending on states of amnesia he's willing to swing his stance on world destruction in an instant, and if he can relate other females (Rouge and Amy) to her, then surely it says that in some twisted way he does have a heart underneath that edgey exterior. I'm not saying the guy would ever be social, but there's a long gap between being a loner and having no reason to live. Shadow's purpose ties to Maria, and it is simply not possible for Maria's pleas to have been made in the context of merely stopping Gerald's vengeance for a death that he doesn't yet know has happened.
Further, I'm not convinced that Shadow knew of Gerald's program from the beginning. Surely he didn't give a damn; Shadow saw humanity as hopeless at that point, but that doesn't say if he knew beforehand or whether Eggman was a means to an end regardless of Gerald stepping in. Surely the Eclipse Cannon is a fine instrument of revenge irregardless of whether or not Gerald's program hijacks it. If Shadow knew all of Gerald's work beforehand, then
loses all significance, and Shadow should not be flinching when he sees the picture of Biolizard, nor should he be going along with Rouge questioning the validity of his memories.
Different characters do matter alot in a game where all you do is press forward and shake your right hand like a maniac. Why improve the standard gameplay when you can add general bullshit to the game by changing the gameplay slightly and adding new skins, by way of characters who don't even have much of a motivation to join you other than that you've just kicked their ass easily and repetitively?
Are you really at all familiar with BK's lategame? Because for someone who rips into me for judging a game on early impressions, comments like these sound a lot more like hearsay than actual experience to me.
First of all, plenty of stages involve a lot more than waggle-fests. There's a lot of platforming action which involves precision jump-dashing and cancelling; every bit Secret Rings' equal. And no, I am not talking about Legacy stages, fun as they are, they're also ludicrously easy. If you don't like the control mechanic of movement simply equaling a button press, that's fine and dandy, but don't pretend that the game stops there. It doesn't.
The motivation of the other characters joining you comes with the revelation of who the real villain is. That's why they're playable only in the game's second half. Kicking their ass doesn't motivate them to change allegiance, in fact it actually leaves them feeling unworthy of living. Sonic smacks some sense into them and leaves them in the dust.
And I don't see how it's bullshit to add characters who, unlike several of Sonic Team's more infamous efforts, do not run major interference with the game's core mechanics to such extremes as they cannot exist without exclusive stages tailored to them, nor do they mandate wasteful hours replaying the same game out of necessity for some coveted unlockable. Are ProtoMan and Bass bullshit characters in MM10? Is Tails a bullshit character in S3&K? There's nothing wrong with alternate non-mandatory characters who do not disrupt the game's focus; it adds personalization and replay value. Something which since Heroes Sega has almost completely forgotten how to do.
Now, can you see a character with his personality being a hero?
It's fairly common to see an anti-hero who happens to fight the bad guys but cares nothing for the typical "hero code of ethics". Shadow was once presented as such in the comics (he's a bit more emo nowadays).
And see, that's the thing. Who you're fighting against doesn't decide whether or not you're the child-friendly hero. In the MegaMan series, we see characters all over that spectrum, from the "good guys" such as Rock and X, to the "fight for what I believe in and don't give a damn what others say" such as Zero, to the "allegedly carries some sense of justice but by all appearances seems to be a rebellious violent psychopath" such as Bass.
Shadow works with G.U.N. for the same reason he worked with Eggman: as a means to an end. His purpose ties to Maria, working with others is simply the best way he sees to get the job done.