Solterra only gains information of importance on individuals he expects to fight, otherwise he feels it is not his business. Solterra tends to fight honorably and respectively, but he feels that the betrayal and disrespect and the wrongful actions commit Solterra to do everything in his power to save his sister

Also, sadly the man doesn't know who his fathers killer is, it could have been Lady Olga. Although, Solterra is an honorable man, if the man asks Solterra, he will admit to the assassinations.