While the Marker/Seal combination in Dead Space 1 was relatively consistent, presumably because it was a fairly high quality copy, the Titan Marker was much more confused, presumably because it was more poorly made. This has caused the two psychic entities to be partially merged, despite their diametrically opposed natures, and thus, essentially, schizophrenic. The real problem arises from the fact that copies of the Marker were made- imperfect, man-made copies that didn't understand the fundamental makeup of the combined artifact, and thus recreated both the original Marker and the additional seals in unison, as a single piece. The psychic influence from the seal is much more complex but narrow- it can only affect one or two people at a time, and it gives them complex hallucinations of dead loved ones that give them advice or guidance. The Hive Mind psychic influence tends to be basic but broad- it makes everyone in the radius suicidal and violently psychotic, so that there are more dead bodies.

There is the Marker's Necromorphic Hive Mind, which wants to spread the Necromorph infection, and then there is what was placed there by the precursors, which wants to contain the Necromorphs. The reason why the psychic influence of the Marker seems so contradictory is because there are, in fact, two distinctly different entities present.

The Dead Space effect helps keep the Necromorphs contained while it is buried, so that none of the bacteria or worms and whatnot that come in direct contact with the marker get converted.

The carvings on the outside, which describe the Necromorph DNA, are basically a hazardous material warning, complete with a detailed description of the contents, and which human researches only tend to be able to partially translate (continually missing some of the important bits, like 'Recommended Containment Protocols' and 'Why you shouldn't let this get loose'). For whatever reason, these precursors couldn't or wouldn't destroy the Black Marker, and instead, they sealed it away on Earth- modifying it with the external carvings, the base, and some other things which create the Dead Space effect and the additional psychic effects (see below). The Marker is Sealed Evil in a Leaky CanSo, however many millennium ago, the Black Marker and it's Necromorph-generating properties existed, and was fought against by some form of precursors.

Dead space 2 nicole concept art