SOA Writers Didn't Know How To Handle Donna's Murder Storyline....
They built it up in the first season and parts of the 2nd season which Jax knowing this big secret and his hatred for Tig and Clay, but then they totally botched it.
-- Opie only finds out because Tig takes some bad peyote on the Indian Reservation and spits out after guilt.
-- Piney makes a half-assed attempt to kill Clay, instead of Opie deciding to get revenge, which was the easy choice. Opie is supposed to be a bad-ass outlaw at this point.
-- They're never even a discussion with the other members of the club about whether or not Tig and possibly even Clay deserve to meet Mr. Mayhem.
-- Opie decides to put all the blame on the ATF Agent Stahl and even in the later seasons he doesn't have any building resentment with Clay and Tig over it.
Clearly the writers wrote themselves into a box with this storyline and didn't know how to resolve it successfully -- so they had the Gemma rape come out so the group turns their whole attention on that, like the Donna murder was the equivalent of Tig smashing Opie's car window.