What caused Bertrand to be the sorta more twisted twin at first than Alec? I mean they are both twisted
By twisted, I assume you mean Bertrand's capacity for evil as manifested from a young age. A lot of factors went into that, but there's a main reason at the core of it: Bertrand simply did not come out of the printer the way Alec did, with the same neurodivergencies and the same mental illnesses, and the ones he got instead caused the adults in his life to respond to him drastically differently, setting off a chain reaction. There's nature and nurture, and while I'm generally a nurture truther, I do believe that nature has a role to play - in this case, through the responses that the people around Bertrand had to his childhood psyche. Nurture responding to nature.
Where Alec was volatile, pushed and pulled by extreme highs and lows of emotion, Bertrand's emotions were always driven more inwards. An example of how this works and how it affected them comes up early on: whenever the two boys were forcibly separated, for whatever reason, Alec would scream and cry and lash out and Bertrand would simply become extremely quiet and disinterested in everything. As you can imagine, this led to Alec being quickly branded the problem child and Bertrand being seen as better behaved, more manageable, more mature - more trustworthy. And I'm sure you can imagine what a boy like Bertrand, constantly dealing with his mother's abuse of himself and the only person he loves in the world, robbed of power and control in any healthy avenue, would do with that trust.
Bertrand learned to lie and cheat and steal and blackmail because it was his only way of exerting control over the world around him, and because - lest we forget that the Underwood home is a brain-eating amoeba in white limestone - it pleased him to make people display the kind of pain and misery on the outside that Bertrand bottled up on the inside. He's always loved to get reactions out of people, while being a very under-reactive person himself. Standing next to Alec, who was a hurricane from the time he was born and who suffered every single day for it...of course Bertrand learned to do things a little differently.