as creepy and incestuous it is, all for one is obviously obsessed with his brother and wouldn't kill him. i mean, I'm pretty sure a big part of why he is trying to get ofa is so he can get to his brother again. he spent a lot of time and energy "convincing" yoichi to join him

i don't think afo considered the 2nd and 3rd as real threats enough to target them, not until they snatched yoichi away from him now imagine that possessiveness with added fury during that, it was afo's prime time, I'm sure it was easy for him to target the 2nd and get him

so phase 2: afo kills the 2nd user, not yoichi, but yoichi being a true hero, sacrifices his powers to give to the 2nd and successfully revives him unfortunately, as we all know yoichi was very frail and weak i think ofa, despite it being in its basis, was one of the

NOW NOW, I know, izuku being the last user is a big obstacle here, but remember they said he MIGHT be the last user, cuz of how strong ofa got, but i think that if it was only partial(?) transfer then katsuki can handle it

of course, there's still a huge possibility that the 2nds quirk will help heal / revive katsuki but i think it'd be beautiful if bkdk paralleled the first and second even more by having izuku save katsuki in the same way (i assume) yoichi saved the 2nd. End.