Beater Concept

The "beater" concept was never really accentuated in the anime adaptation; as we know, a beater is a mix of the words "cheater" and "beta tester"; SAO's beta testing was very esoteric, not a lot of people got in -- but those who did get in, had a huge advantage during the game's launch. SAO wasn't designed for beginners to easily grasp the basics; it was a game of trial and error, deaths were integrated to be commonplace [SAO was the first VRMMORPG; commands are drastically different in comparison to traditional systems]. Given this, a player would probably "grasp the basics" of SAO after a decent portion of testing and error [perhaps a few hours, or at least a few levels to grasp how combat functions, sword skills, etc]; but, SAO's release was much different than the beta -- there was no log out, and deaths paralleled in severity to the real world. Given this, numerous players died on launch day because they were oblivious to the functions of the game; the beta testers [which in most cases, are methodological gamers; not dilettantes to the gaming genre] followed their instincts and used their beta testing to their advantage [during most game launches, common sense is to get ahead -- the beta testers in SAO did just that]. So, Kirito, being one of the beta testers, was blamed for essentially allowing the majority of the players, the non beta testers, to die.

Of course when we view this pragmatically, it isn't Kirito's fault. It's not a "beater's" fault at all for the lack of knowledge that most players didn't have. They weren't obligated to help them. But, we do have to keep in mind that they don't have to be "right" in order for Kirito to feel immeasurable guilt. The beta tester to non beta tester ratio is incredibly skewed, perhaps 1:99; there wouldn't be much room for argument. It's comparable to modern-day politics, where the common folk blame the "elite" for their problems [highly generalizing politics here, not a tea-party enthusiast yo]. Life in SAO is incredibly precious; the recent episodes may not imply it, but death didn't happen incessantly. We probably saw <25 deaths throughout the entirety of SAO, and that's over the period of two years [keep in mind, SAO's adaptation covered numerous dungeons, these are dangerous, "high chance of death" zones; death wasn't widespread]. So, when death does occur, it's ridiculously significant. That was the case with Kirito's situation, regardless of verisimilar rationality, he believed that he caused the deaths of thousands of people for being simply himself, an objective gamer.

Sachi's death serves as an antithetical point to the "beater" tag. He did little to nothing to actually deserve the "beater" tag, so he could have shrugged this title as nothing more than superficial, frustrated denunciations by the general populace [who were wary of the imminent chance of death]. But with Sachi, he had an actual chance to save her; her death was the direct result of his actions. The 'beater" tag is not a result of his  direct actions. After Sachi's death, the already ambiguous bodies of "What is my fault" and "What is not my fault" crossed; he viewed the "beater" tag as holy, considering that he truly is abhorable [from his eyes; he allowed Sachi to die]. If Asuna was not introduced, and served as a cathartic, symbiotic partner, he would have inevitably gone insane, or would have broken down.