Chapter 1, Halacha 4

Chapter 1 Halacha 4: When the Rabbis say lashon hara is worse than the big 3, idolatry, adultery, and murder, and not only is one punished in this world, one has no share in the World to Come, they are referring to someone who habitually speaks lashon hara, a baal lashon hara. This person might have known lashon hara is a sin but now he doesn’t even realize he is doing anything wrong because he speaks it so often. This is worse than the big 3 because, for example, if one is sentenced to death for murder and before he is executed he admits he did wrong and repents then he gains his share in the World to Come but this person who is constantly speaking lashon hara and not even thinking about the ramifications of what he is doing or saying is essentially rebelling against Hashem without any remorse. The fact that he doesn’t think he is doing anything wrong and has no intention of admitting his guilt and repenting make it worse that the big 3. 

Chapter 1 Halachos 1-3

Chapter 1 end of halacha 1 – 3

End of halacha 1: There is a discussion amongst the Rishonim whether lashon hara and rechilus are all part of one verse of לא תלך רכיל בעמיך or is rechilus (tattletale) is worse than lashon hara (slander) and lashon hara is learned from לא תשא שמה שוא and although we should not need a verse for rechilus because we can learn it out from a Kal vachomer (fortiori) from lashon hara however the Torah goes out of its way to have a separate verse for rechilus in order so that the court can give lashes to one who falsely slanders someone else. Another ramification of the extra verse “Don’t walk as a tale bearer amongst your people” is that the prohibition starts even before one actually speaks lashon hara rather when he is walking to do the sin it starts. 
Halacha 2: A reminder that other prohibitions like revenge,hating someone in one’s heart etc. can be transgressed while speaking lashon hara. 
Halacha 3: A person who habitually speaks lashon hara is in a whole new realm called a “baal lashon hara” he or she goes around collecting info about people and then sits around with a crowd talking slander every day. This type of person is viewed as someone who spites Hashem and his Torah because he doesn’t just sin every once in a while but premeditatedly sins every day by creating groups of shmuzzers to speak loshon hara and rechilus.