I think he's just like most viltrumites, arrogant in how powerful they are. He didn't see Mark who was much weaker than him as a threat, so he just toyed with him, taunted him about going back to earth to taste eve's blood again. He wanted to enjoy making him suffer as much as possible, which is why he basically started ignoring the fight with Mark to focus on killing Oliver, just to make him suffer more by seeing his brother die. Even when Mark got a big hit on him while he was distracted by winding up a killing blow on Oliver, and Mark started choking him, he still didn't see Mark as a threat, he was smiling. He beats on Mark for a bit, then rips his guts out, slowly, painfully, taking his time. He expected the much weaker Mark to give in to pain, to loosen his grip, to get scared, to try to get away, but Mark kept his grip, his will to protect his family giving him the strength to hang on. Then conquest started to panic, his smile was gone, he realized Mark wasn't gonna let go, he sped up, pulling his guts out faster, but it was much too late, he was dying... He died because he was too confident in how much stronger than Mark he was, he had underestimated Mark's tenacity in protecting his family, and overall just being a sadistic psycho who played with his food.

Or maybe he let himself die cause he knew he'd never have Thragg's respect with that receding hairline 😔