This is my theory of how the dementia works.
The Two Powers
It may be so that the Marker acts as the "good guy" and the Hive Mind acts as the "bad guy" in Dead Space.
Isaac was under the influence of the Red Marker, which told him to "Make Us Whole", referencing to the Marker and the Hive Mind. That must mean that the Marker wants to re-unite itself with the Hive Mind, puting it in a dormant stage. However, the Hive Mind wants to have control over his "Minions" AKA Necromorphs, and is unwilling to let that happen, which is the reason why it makes the Necromorphs kill people, stopping them from returning the Marker. In my theory, the Marker and the Hive Mind are opponents, both trying to sieze control over the other. The Hive Mind uses its own telepathic powers to make people kill themselves and others(this was the case with Mercer) if they were out of reach, for example. Since both the Marker and Hive Mind want control, they "compete" over the people. As a result, both of their powers are taking control of the victims. This causes them to go insane, because of the mixed message the Marker and Hive Mind are sending. This would explain why people have such odd behavior. Some seek to kill themselves or other people, such as Hanson in Dead Space: Downfall, where he just stops outside the elevator (obviously influenced by the Hive Mind, wanting more Necromorphs), and others try to re-place the Marker on the planet, such as Dr. Kyne and Isaac. They can not, however, control everybody at the same time, which is why some where not influenced by the dementia until after a while, when the rest where either dead or unable to take control over, such as Lexine. The reason why Necromorphs can't get close to the Marker or persons under heavy influence of the Marker, would be because of its opposing energy being too strong.
I have two theorys for the events of Dead Space 2:
Theory 1: In Dead Space 2, where there is no Hive Mind for the Marker to compete with, the Marker feels it has control and wants to have more, meaning the Markers are able to have certain "personalities" concidering it is a new Marker.
Theory 2: Tiedemann has managed to create a "corrupt" Marker that helps the Necromorphs. One of the things pointing to this is the fact that there is no visible "dead space" field around the Marker, seeing as how the Necromorphs where able to approach it without any difficulty. This also means that different Markers have different "personalities".
Well, the government finally created something that actually has a good purpose, but who is the fool; the guy who makes the weapon, or the guy who copies it, fires it and kills hundreds of people, and then keeps it a secret!? All that text on the Marker and none of them are warnings! Or they are, but the scientists just figured they didn't need to check! Who knows?
Does it sound reasonable?
"Unitology is a lie... and so is the cake!"