Marcus at some point in his life became a gangster. He eventually hired Clay Parker prior to being imprisoned. He saw what Shawna can do with her powers after her capture. He along with Clay decided to skip town to avoid the Flash.
Like most gangsters, Marcus is cold and ruthless, as he does not like people in general losing his money. Even after Clay Parker, paid his debt to Marcus, he was still willing to kill or injure Shawna Baez, Clay's girlfriend, possibly to make an example of what happens to those who lose his money, in a cruel or sadistic way.