Why do i feel feel sorry for him, if i now hate him?

ok, well.............i have a friend that recently....has been idk. but i am literally HATING him. ever since high-school started everything has changed. i used to be the weirdo outcast, and he has been mr.everyones friend. but now its exactly the opposite. he now has gotten fat, has a video-game addiction, most likely into drugs, and EVERYONE thinks he is the weirdest person alive. i dont know why im feeling so sorry for him. i feel like i should be were he is. i can handle it (dont give 2 ***** on what people think) he docent handle it vary well. the the way he handles his situation is just lying to himself. i really dont like the new things about him. i hate fat people i hate people who play video_games all day, i hate people who do drugs and i hate lying! there is also this thing with his sister -_- . he has just become....gross. no one wants to be around him. EVER. except his two friends. that is including me. his other Friend has become kinda my best friend. to be more specific. we recently had a campfire at his house. we were having a great time (his sister was there with her friend) but whenever he said anything it got so, so terrible.when i mean terrible, i mean whatever came out of his mouth was.... so blaahh!!!! i always feel like beating him up. i would to anyone else, but he is really weak. but i also feel so sorry for him. and there is always the thing with his sister. what should i do about him? im about 2 explode! i hate his denial! if i do anything to "hurt his feelings" his sister and my best friend might be over me. or if i Stop being friends with him the same could happen. and am i only friends with him because im still with his sister? or do i still want to try and help him? we have been friends for years. i really need some help with this. what should i do?