[hinode]

There was definitely a "nice guy is friendzoned by cute girl who chooses charming asshole over him" love triangle quality to Kariya/Aoi/Tokiomi, but that so-called summary is so damn biased against Kariya that it might as well have been written by Zouken.

Heh, you missed the point of the Nice Guy Syndrome analogy completely (although that definately sums up Kariya's view of the whole relationship!). Internet Nice Guy Syndrome (see here and here) is about self-proclaimed "Nice Guys" who are in fact in fact anything but when dealing with women.

Kariya's intentions towards Sakura are noble and pure throughout the entire Grail War, but his behavior towards Aoi is seriously messed up. He desperately wants to believe that he's her white knight savior sacrificing his body for her sake, when in fact if he ever bothered to actually listen to what she has to say he'd realize that he's completely trampling over her actual feelings (as opposed to what he imagines them to be); for Aoi, the concept of a deathmatch between him and Tokiomi to be a horrifying no-win situation.

As the war drags on further and further and Kariya's sanity drops further and further, he increasingly projects his wishes onto a distorted fantasy version of Aoi that he develops in his mind. Ultimately, his inability to recognize the vision of Aoi in his head with the real thing is what causes him to snap and start strangling her.

The LN version of the event, spoiler tagged for length:

Spoiler:

Bending her knees to the floor, Aoi lifted Tokiomi’s face up. With no place to hide, Kariya could only watch her. He could not understand why Aoi did such a thing. – No, he did not want to understand. Why she did not even spare a glance at him, her childhood friend, but continually fixed her eyes on the body of Tokiomi; what the meaning of those tears falling down her face was – Kariya obstinately rejected comprehension, and because of that, he could not utter a single word.

If memory served – he was supposed to never make this person – whom he loved above anyone else – cry again, and to fight for her even at the cost of his life.

If that was the case, then this woman sobbing in front of him right now – who was she? If he were to accept the answer, wouldn’t Matō Kariya fall to pieces?

She did not look at Kariya. As if ignoring the air and all else, tears poured out from her to her husband’s body. She, the tragic heroine, was exactly at the center of the world as its axis of revolution. Having regarded her in that way, Kariya was an existence devoid of all meaning, like trash on the stage or a stain in the background. Kariya was terrified of the delusion that his position and his very existence were erased. He even felt the impulse to scream to catch her attention. But not a sound could come out of his dry throat.

Nevertheless, before long, when he was looked straight in the eye by Aoi – who had finally lifted her head up – Kariya finally understood: Ignorance was a greater compassion; if he could disappear from the world at that instant, it would be a far greater relief.

“……So now it’s as if the Matōs had gained the Holy Grail, right? Happy now, Kariya-kun?”

It was a familiar voice, but in a tone he had never heard before. That was because his gentle and kind childhood friend had never hated or cursed anyone in front of Kariya before.

“I – but, I – “

Why must he be blamed? Tokiomi Tōsaka was the cause of all the evil. When that man was not around, everything was supposed to turn out well. First of all, why had he died in a place like this? It should have been Kariya asking questions instead.

“Why…..?”

However, not even allowing time for Kariya to reply, she asked him back instead.

“So after snatching Sakura away from me, the Matōs are still not satisfied? Of all people, you killed this man in front of my own eyes…… Why? Did you really hate us so much?”

I don’t know I don’t know.

With a face just like Aoi's; with a voice just like Aoi's; why was this woman directing such seething enmity and cold killing intent towards Matō Kariya?

Kariya was supposed to have saved Aoi. He was supposed to have obtained a future for her beloved daughter. Then why was he being blamed? Who on earth was this woman?

“That guy – it’s his, fault – “

Pointing at Tokiomi’s corpse with a shaky hand, Kariya tried to correct Aoi in a loud voice.

“If that man would just, disappear – no misfortune would befall on anyone. Aoi-san, Sakura-chan – I thought you’d be, happy –“

“Stop talking nonsense!”

With a demonic expression, the woman shouted.

“You, what do you know! Someone who has never loved anyone before!”

“– Ah –“

With a snap –

– the decisive cracking sound caused Matō Kariya to collapse.

Of course this is all ultimately Zouken's fault in the end; as messed up as Kariya's behavior got, he can legitimately plead worm-induced insanity for everything he did. The only thing you can really fault Kariya for is actually agreeing to a deal with Fate's resident devil to essentially become the latter's puppet, subjecting himself to endless misery and torment for ultimately no benefit.