Somehow I get the impression that this girl was simply not worth your effort in having a go at her. It is very juvenile to want sex simply for the pleasure it gives and sooner rather than later the relationship blossoms into something like love. If the fat guy has found this, well good luck to him! you probably would not, at least not here.
Killing him will not do you any good and the revenge that you seek is no reason for taking life. Revenge is not sweet, but will lead to greater dispair when in later life (in jail) you appreciate what you have done.
What you are expressing is what we all feel, when we have lost out on a project that looks to be good, but in practice may not necessarily turn out so well. You suggestion to get some money from the girl's father reads like you were planning to steal it or get it whilst pretending to love his daughter! This attitude does you no credit, grow up!