Eh, fair enough. DT seemed to be willing to do shit for money (merc) but was also big into perfecting their craft - i.e. not just learning how to imitate a person's speech and mannerisms but also learning what made them tick - method acting them.
So yes, torturing Catra for giggles seems more like a plot directive than a character-driven one - and a poor choice for a mercenary who wants a regular payday.
But I also don't find it especially out of character for DT since they don't care about anyone but themself. It does establish them as a shitty, shitty person though.