I think part of the problem is that there were things done for writing and pacing sake.
If Gareth hadn't walked in and interrupted, Rick and the boys would be dead simply because of how fast the butchering goes, well before Carol would have started.
Additionally, Carol's activity is what gave Rick the break to strike. if Carol didn't exist, or had been killed while on her private journey, things would have been different.
Technically, all these things happened perfectly because the writers chose to make it go that way. All things written are subject to this, so meta-gaming about how it turned out is kind of loopy.
What remains then is what the writers appeared to wish to show us or inspire in us.
They wanted us to hate Gareth and the Termites, and showing us how evil he was with is just business demeanor, as Conan said, he was once a manager at Starbucks.
They wanted to move on past a single set location (unlike The Farm or The Prison). So after the many episode build-up to Terminus, they had a quick reveal and resolution. I expect now to see a number of 'travelling" episodes as the group makes their way to DC.
Unless Gareth goes after them, or the group stays near, we won't see the Termites again.