You're deliberately ignoring the point that the primary use of a semiautomatic pistol is killing people, whether self-defense or mass murder.
Should you be culpable for giving your brother an extension cord if he lies that it is for the porch? Not really.
Should you be culpable for giving your brother a gun if he lies that he needs it for self defense? IDK the answer, but it's absolutely not equivalent.
It is a higher level of responsibility, you know lives are in danger if you give them a tool for killing. I don't think it's unreasonable if there is a higher standard for loaning it out or leaving it unsecured.