Im thinking the word got out that the Aryans were taking orders from a rat. So then thats how the black dude gets involved, Otto gets payback and the Aryans lose respect. That made more sense then them not killing Zobel.
Also they are standing on a dock and Jax kid just got kidnapped. 1. They can call charming PD to call coast guard and scoop up the kid. 2. They could of hi jacked any boat they saw people on and chased dude down.