Remember how Tarrlok’s eyes widened when he heard about Amon? Since now I don’t think he anyhow works with Equalists, although Amon’s revolution is really useful for him for gaining more power. And I also think he is scared of loosing his waterbending.
On the photo we see Yakone’s (if it’s him) eyes widen because of some reason. At first I thought he got mad and started bloodbending with his eyes (LOL), but after making a correlation to Tarrlok’s eyes, I came up with the idea that Yakone was sentenced to taking his bending away and that made him so mad.
But he also could be bloodbent by Katara or someone else (looking forward to see grown-up Katara).
Anyway, Aang would take his bending away, so he would never be a thread for Republic City.
Probably it could be Yakone who tought Tarrlok how to bloodbend. Even without his abilities, he knew the technique and theoretical part. While teaching him, he also would tell his story and how terrible Yakone’s life became after loosing his abilities. That would make Tarrlok so mad at Equalists and afraid of loosing bending.