BUFFY SEASON 8! CONFUSION! QUESTIONS! HELP. (spoilers)

Hey guys so I just finished reading the season 8 of buffy and I'm so confused about.. well everything. Can someone please explain this season to me just the main stuff? I was understanding most of it but then it all just went over my head when it was revealed that angel is twilight..so was angel the bad guy? or not? if he was then why did buffy have sex with him? didn't he kill bunch of slayers? what was that mark on the chest all about? how the hell did spike know what was going on? I'm just very very confused?? The rest of the slayers didn't lose their powers right? and did Buffy still had the superhero powers so were they destroyed when the seed was destroyed? and giles died? what? what happened to angel? what. Someone help me soon or my brain will freaking explode. Thanks!