I must say it, this last episode has drven me crazy about this question, during the 42 minutes that it lasts I've changed my mind about this about 4 times!!and I'm still in shadows! the facts lead to both drections and no one is conclusive! I've made a mental list of the evidence and what does each evidence makes me think of
1) Ben killin' locke afte he noticed he knew about Eloise,and seeming surprised
to see him alive --> Bad's the Bad guy (B.G.)
2) Ben turning Caesar against John by lies--> Ben B.G.
3) Ben Killin Caesar tells me that he led to him in order to gain his trust--> Ben Good .G.
4) Ben not knowin' why is John alive -so killing him with the idea of not seeinmg him anymore---> Ben B.G.
5) Charles wantin' Ben to be left dying, and then wanting both baby Alex ad her mother dead,
and the surprising (yeah ,surprising)decision of Ben of saving both lives --> Charles B.G./Ben G.G.
6) Ben having doubts when trying to kill Penny ---> Ben G.G
I'm probably forgettin somethin' , but my provisory conclussion is that Ben is the Good Guy, and the bad guy is Widmore
WHAT DO YOU THINK ABOUT IT?? WHO'S THE REAL BAD GUY?