But "security and ease" are not teh same as a special pleasure in "going in for the kill" with vulnerable girls... He may feel that if a girl is easy to seduce he does not have to trouble his head about her, since she is willing to go ot bed with him but that is not IMO the same as saying for example, "here's a penniless servant girl who can't complain about me, so I'll rape her..and enjoying hte fact that he has forced her.
Of course, selfishness and weakness can ruin lives, but that does not make everyone who is selfish and weak a total monster....
Willoughby does not need a "victim" to get pleasure necessarily. There are plenty of ways of getting pleasure that dont need a victim.. drinking, gambling, sex with willing and easy women, sport (unless you consider the animals to be victims). To be honest, since I don't think he intended to seduce Marianne nor that she would be so foolish as to yield to him, so in a way, I dont see what he got out of the relationship, unless he was genuinely attracted to her and wtih at least a part of his mind he was considering marriage...I think it HELPED that Mrs D was so foolish that she didn't haul him up after a short time and ask his intentions... becuase that way he wasn't forced to make his mind up -. However that aside I cna't see what he got out of the relationship other than basking in Marianne's admiration for a while....