any yaoi manga about a weak uke but when he snaps he will be really cruel and strong then ...

[Commenting because I'm interested]
I'll try looking for some as well ^-^"

http://www.mangago.me/read-manga/senya_ichiya_shitone_no_himegoto/

a bit more complicated (art is ugly but its a good one)
http://www.mangago.me/read-manga/negai_kanae_tamae/

....not exactly what you are looking for:
Ken from
http://www.mangago.me/read-manga/sabita_yoru_demo_koi_wa_sasayaku/
he is really cruel and abusive, and then he ends up as an uke here
http://www.mangago.me/read-manga/renai_rubi_no_tadashii_furikata/bt/ebd618fe1b9ef76dd5ee458671769b51_chapter-4/pg-1/

Sorry... did forgot the weak part... they are all quite strong but crazy...