The main problem with Terrance is that he is just one of the worst examples of an “abusive sibling” trope who does is just to abuse Mac, his little brother, the entire time whenever their mother is out at work.
Despite being the series’ “real main antagonist”, he is absolutely one of the worst, yet insufferable characters as mentioned above and below.
At the beginning of "House of Bloo's", he beats up Mac and Bloo while using them as weapons to wreck the apartment, which was incredibly violent and abusive, and such actions could land him in jail or juvenile detention for that. Not to mention that he also got Mac blamed by their mother, who was also a jerk in the said episode, and got her to force Mac to get rid of Bloo while Terrence was merely asked to go to his room. You you look closely at Terrence's evil grin after hearing Mac's mother forcing Mac to get rid of Bloo, he got what he wanted and got away with his mean-spirited behavior scot-free.
He never got punished for abusing Mac and Bloo, or for wrecking the apartment in the first place, which might've sent a really bad moral to children saying it's okay for big brothers/sisters to pick on their younger siblings.
He teamed up with Duchess, and neither of them got punished hard enough for trying to kill Bloo with the Extremeasaurus.
He locked Mac up in his own bedroom closet, which was also abusive of him.
His dialogue is incredibly painful to the nerve to and makes no sense such as his “I just want to punch you!” catchphrase whenever he encounters or plotting against his brother and his imaginary friends, but what on earth would possibly his mother or someone treated him like that!
He was at his absolute worst in "Seeing Red" as he bullies Mac once again throughout the entire episode, which shows that Mac's mother forcing Mac to get rid of Bloo didn't stop Terrence from abusing Mac. Here are some other examples of his outright cruel actions:
In "Duchess of Wails", he teases Mac about going to Singapore after their mother threatened to move out, which horrifies the latter.
In "Infernal Slumber", he threatens to tell his mother on Mac, and if Terrence's plan to get Mac and his friends from Foster's in trouble was successful, then their mother would've forbidden Mac from going to Foster's ever again.
In the series finale, "Goodbye to Bloo", he drops Mac's stuff onto the floor and breaks it, which made Mac angry.
He never got arrested for his violent actions towards Mac for years, which is a sign of him being a Karma Houdini.
This is probably because of moral guardians not liking seeing underaged characters getting arrested on TV and that he and Mac's mom wouldn't be happy with her son in jail.
As if all of the above mentioned bad qualities weren't bad enough, what makes him especially infuriating is, much like Eddy's Brother (from Ed, Edd n Eddy) and Micah Brill (from the Goosebumps book Revenge R Us), he's never given a good reason as to why he bullies Mac other than for his own amusement.
Just like his and Mac's mother, the show wouldn't have happened without his cruel nature.
He does get some comeuppance at the end of "House of Bloo's", where he was cornered by a Pegasus herd, and also in "Seeing Red" when Red, the imaginary friend he created threw him away in the dangerous traps Red himself went through with Bloo.
He gets grounded by his mother in the end credits sequence of "Infernal Slumber".
He did have some funny lines, such as "...cause I know how stupid I...", which almost got Mac and Bloo laughing so hard.
The only episode he was tolerable in was "Berry Scary", with him innocently changing the TV channels.
He was likely intended to be unlikeable to begin with.
Much like all of the other characters, his design is creative to look at.
Tara Strong does an awesome job voicing him.
Tara Strong once said in an interview with voice actor Rob Paulsen that Terrence is her least favorite role she's done because it strained her vocal chords.