[Guido]

Way back to 2013 when season one was airing I expressed my thoughts that I should not rush at judging Annie, Reiner, and Bertholdt for what they did in Paradis Island due that believable characters in order to be relatable they must possess a certain type of multi-faceted and multi-layered personalities. That means that their motivations for doing things cannot be childish nor cartoonish.

Hence, this episode delivered the needed and grand, pay-off for Reiner's group. After all, Attack on Titan has been recalling the audience that we are all flawed humans able to disagree at moments and later on to come to agreement at dire times, though not necessarily successfully.
Labels such as heroes, villains, and the such are just tags or qualifiers, subjective and abstract and relative in the greater scheme of things.

While Eren has come to understand Reiners' motives for what the latter did and empathize with his former comrades' pleas, he could not let slide what Willy Tybur proclaimed on-stage: they're all people who do not wish to die, because they were borned into the world and cannot help it anymore.
Therefore, Eren has steeled himself clearing all distractions and doubts from his thoughts, that he's resolved to slaughter the enemies no matter the cost because no one has left him a choice. In fact, I'm pretty sure that Eren already understood such conclusion four years back realizing that choice was just a fleeting 'dream' and can do nothing else but rampage his way forward until Paradis gets freedom even if that means literally waging war against the whole world.

For Eren, his truth is clear: it's either killed or be killed; an unfortunate reminder that humans are forever shackled in the shroud of nature and cannot escape it rather cope with it.

Now, the time has come for the starving wolves returning the favor with a big comeuppance upon the pigs; the nine-years wait for Paradis's retribution is officially over as Eren's attack titan replies back with his own declaration of war to the whole world.