This kinda makes sense. Masters was "betrayed" by Jack when he got that blast from the mini-portal, had to go to the hospital for ecto-acne, and got his social life killed (according to Jack). Masters probably felt like the world was against him, so when he got these ghost powers he decided to use them for his own personal gain, either by robbing banks or overshadowing people to give him stuff. And when he goes through the Ghost Catcher, he still has that mentality.
Where Plasmius was just formed in this man, either during the accident or sometime during the years in the hospital, and he doesn't know why this man is hurting and wanting to get revenge, all he knows is that his human counterpart is using his ghost powers for evil. And it only got worse when Plasmius finds out that Danny is just like him, but Masters only wants Danny, a 14 year old boy who's just got ghost powers a couple months ago, to join him in using his powers for evil. So when he gets separated from Masters, he wants to help Danny stop Masters from commiting crimes, and also teach Danny how to use his powers, since he is the ghost half of a halfa whose had powers for about 20 years.
When he pushed Vlad through the Ghost Catcher, Danny found out something surprising and not so surprising in hindsight.
What wasn’t as surprising was that the human and ghost halves of Vlad fucking hated each other.
What WAS surprising (to either Danny alone or both Danny and Vlad) was that Plasmius was the one preventing Masters from committing- well, committing more crimes and atrocities. Not the other way around.