They're both examples of someone who may not be morally upstanding, but who was worsened by the decisions of the people around them.
The I-Rex was raised by people with no idea how to provide enrichment for an intelligent reptile, so she ended up worse than a standard "hungry and big teeth" dino. No one intervened, and everything went terribly murderously wrong.
Likewise, Thresh was in a situation where no one knew how to deal with a budding sadist, so he ended up worse than a standard "jerk who's at least good at his job" warden.
And the I-rex eating her sister is a parallel to Thresh being cruel early on. Namely, there's blatant evidence that this individual's got problems, but instead of doing anything to fix that,the relevant authorities just put said individual in a paddock/magic basement alone. Then the individual gets worse and cue havoc.
Let me be clear that I'm not trying to excuse Thresh's actions any more than I'm trying to excuse what the Indominus Rex did. They were both huge threats to everyone around them, and they both needed to be stopped. But in both cases, they could have been stopped _before_ they became such huge threats.
(Credit for the Thresh analysis goes to my discord buddy Skull Friend/LeviofEnvy.)