Later on, after Thragg kills Thaddeus, and launched himself at Nolan, he tells Nolan that he "Gave him an chance for forgiveness, then an honorable death".
I feel like Thragg actually tried to kill Nolan here with an full powered punch, after Nolan made it clear he wouldn't rejoin the empire. In Thragg's mind it would have been an honorable, quick, and painless death.
However Thragg just underestimated the endurance of Nolan, which seems to be greater than Nolan's strength (something that Mark inherited).
Then when Thragg is beating down Nolan for the destruction of Viltrum, Thragg is again unable to kill Nolan fast, and honestly from the context when Thragg says "I killed Thaddeus to quick, I wont make the same mistake with you." was just Thragg saving face for being unable to kill Nolan the way he wanted.
Finally, after Nolan breaks Thragg grips, and then sends him flying, Thragg gives up trying to beath Nolan to death and just goes for hos stomach, the "weak spot" of Viltrumites because their ribs dont cover it.
I just find it funny, Thragg is atleast twice as strong as Nolan, but imagining Thragg screaming "Why wont you die?!" in his head for the duration of their fight.
Its basically the same thing for all of Mark's enemies lol. They keep kicking Mark's ass, but just cant land an fatal blow.
The Grayson's truly are "invincible" lol.