SPOLIERS, DO NOT READ!
I feel like… Jean should die. It’ll be the most emotional death in my opinion.
And okay! puts on Hanji glasses
Well, the Historia family is known for having powers that manipulate the human mind, causing memory lose… The Beast Titan has this power, but… How ? Grisha goes to the family mansion, and basically killing the entire family. I believe Grisha and the Beast Titan were once friends, and they were the ones that created the thing I’m just calling “Titan Juice”, which you know… Turns the user into a Titan.
Now, think… Maybe Grisha wanted to do… Good with the Titan stuff, possibly something for military? Who knows. And the Beast Titan wanted evil… Seeing as how they wanted different things, he brainwashed Grisha after knocking him out and leaves him near the walls… Maybe this is why he is so interested in the human anatomy.
As he gets older, he regains memory, thus causing him to go and kill the Reiss family ( I meant that, not Historia. ), realizing that maybe his old partner was working alongside them, in which he learned the power to mess with human mind.
Also, remember this! Grisha ate the Titan human shifter that had the Titan power of invincibility, but she was too young to unlock that power, and we already knew he’s a Titan… We also know Eren ate his Dad, thus giving him that power as well, so Eren has the hidden power of invincibility that may help him in the future! Don’t forget that Grisha purposely got eaten… Makes me think that he knows he can’t do anything himself, so he believes his own son will be the one to stop the attacks, specifically the Beast Titan… In the end…
I believe the Beast Titan is the reason Reiner, Annie, Bertolt, maybe even Ymir and the Reiss family are all titans. Hell, he may even be the creator alongside Grisha!