In Underworld: Awakening, Lida, the human scientist at Antigen in charge of taking care of Selene's daughter, was really devoted to her well-being, more than you'd expect for a scientist working for a biotech company on ethically questionable research. It seemed to this troper that the character was sort of unrealistic, then I realized that she probably had a lot closer relationship to "Subject 2" then we are told in the film. Do you think Antigen would have kept Selene out of cold storage for 9 months, even heavily drugged, for her pregnancy? They would have used IVF and implanted the embryo in a surrogate. Given how secret they kept the research, they would have chosen someone as a surrogate mother who already knew what the child was and could be trusted not to tell anyone. Thus, the obvious choice is one of the scientists in the research program; Lida may have actually given birth to Selene's daughter, so the reason she cares so much is that she sees "Subject 2" as her own daughter. ~Nat Fre
The nature of blood-bourne disease that creates "immortals" is somewhat contradictory:
Selene is exposed to Michael's lycanthropy-infested body fluids, but does not become a hybrid herself, yet when she drinks the blood of Alexander Corvinus, she becomes immune to sunlight.
William Corvinus attacked and killed numerous vampires with his claws and teeth, consuming their body fluids, but never becomes a hybrid.
Markus Corvinus seems to gain the memories of any creature whose blood he drinks, in any amount and in any situation, yet no other character does so. Lucian bites Michael and implants his memories, but the only other case of "transmitting" memories was when Selene awakened Victor.
It's more than Markus. Selene explains that only elders have the ability to properly arrange their memories into a coherent history. It's what she was attempting to do when she awakened Viktor, with less than perfect results, and in general, it's implied to be the way the elders manage to maintain consistent rule, despite their two hundred year bouts of hibernation. Michael's fragmented memories from Lucian are more likely a side effect of being turned rather than anything to do with blood, as seems to be the case with vampires.
And as for the rest, we could probably mark it down to "the Corvinus bloodline is weird and not all of the rules apply" since Markus and William are the progenitors of vampires and werewolves, respectively, and Alexander, as their father, probably shares at least some of the traits.
The Cleansers William killed were humans, not vampires.
And on a separate note, from the beginning, it was stated that Vampires and Lycans are both biologically compatible, and capable of breeding. In six centuries or more, why didn't Lucien just capture a female vampire, and get her pregnant? Or, just in more recent times, use artificial insemination? Hell, if he was squicked over the idea of rape, he could have used vampire blood on a volunteer. Instant surrogate mother.
In a non-canonical novel, it's claimed that the reason Sonya became pregnant with Lucians child is because they're both purebloods (i.e. born immortal).
Sonja never came into full-terms with the child so we don't know if it would have survived.
Marcus apparently gets a chance to kill all of the children he doesn't want every two centuries, with the only people close to strong enough to fight him lying in torpor. And, by vampire tradition, also completely dependent on him for reawakening.
It's not so simple. He may be strong, but he's not indestructible. The Death Dealers were all loyal to Viktor. They managed to subdue William in his beast form. They would have had an easier time with Marcus, especially in later centuries with more advanced weapons. Also, Marcus didn't want to jeopardize his chances of finding his brother.
By rights, the 'blood borne disease' thing that makes vampire/werewolf hybrids impossible should be lethal to both biter and bitten.
Viktor's internal Masquerade is dependent on very powerful Markus not getting cranky during his 100 year rule segments. While Viktor has taken care of the key to William's prison fairly well, that does not go for Markus deciding to shatter everything else.
When Markus finally does go ballistic, his father Alexander barely raises an eyebrow.
He was likely biding his time. Viktor was the only one that had any knowledge of the key or the location of the dungeon. Remember, Lucian's escape occurs during Viktor's reign, which wiped out a significant part of the vampire population - or at least those related to the high council. Markus would have awakened two hundred years later to find himself dealing with an entirely different coven loyal to Viktor and Amelia. It wasn't until Markus became a hybrid that he was was physically powerful enough to take out the entire coven himself. Regardless of the events of the first movie, things were bound to come to a head soon. With modern technology and biological science at his disposal, Viktor would have eventually come to realize that Markus's insinuation of supernatural heritage was a deception that no longer had to be entertained.