Alright, I think I figured something out regarding a backstory for your Makuta:
Your Makuta was the master of Destral's Rahi stables and thus directly overseeing the distribution of Rahi throughout the universe. It also was his job to make sure any personal Rahi projects of Makuta were taken care of.
All in all your Makuta did his job well, but he also was greedy and had a weakness for gambling which led to him often visiting Stelt. If someone in the Matoran Universe with enough money wanted a certain kind of Rahi (within limits) your Makuta was happy to provide them with one or more specimens. He tried to keep this secret from his brethren, but at least the higher ups had a rough idea of what he was doing - but didn't care as long as he knew where to stop.
One fateful night in one of Stelt's gambling dens your Makuta met a trader who showed him a valuable artifact and offered to trade it for some samples of the viruses the Makuta use. Your Makuta initially refused, since that was certainly illegal by Brotherhood guidelines, but when the merchant added a big sum on top of the artifact and suggested a bet - the artifact and his money against a small chest of Makuta viruses - your Makuta couldn't resist.
Long story short - he lost the bet and had to give the viruses to the merchant. Your Makuta knew that Teridax would be far from happy about this incident, though, and thus decided to get the viruses back before Teridax learned about it.
While in the end he did manage to retrieve the viruses, his non too subtle methods resulted in a small massacre, though, news of which of course reached Teridax. The Brotherhood's leader indeed was furious at your Makuta, first for being so dumb as to give viruses away, and second for causing such a large fuss when the Brotherhood was yet supposed to look benevolent.
As a punishment your Makuta was sent south, into the unexplored regions of the universe, to explore them. He never returned, however, and Teridax later discovered his remains, confirming his death.
I'll give you until Sunday to upload your Makuta.
Rules and list in post 1
There are 2 Makuta to be assigned again