Alex's death

Ben's most definitely talking about Charles Widmore. Widmore is Ben's primary rival along with Locke, I suppose. Jacob and Man-in-Black (who The Others, sans Richard, only know as the monster) do not lay their poker hands on the table, and Ben doesn't really know them for the bulk of the show. We see in subsequent seasons where he plots the death of Widmore's daughter, Penelope, as revenge. The difference in Ben's mind is that Widmore had a daughter with an outsider (non-Others) off-island, whereas Alex and Danielle Rousseau were brought to The Island. No babies are typically conceived or born on the island due to "the incident" where the radioactivity causes pregnant women to reject the fetus and die, so Alex is seen as somewhat a miracle baby and quintessential "Other" despite being kidnapped. Even despite that, to kill any of your people, is simply against the rules set forth by Jacob, and typically results in banishment or death. The Others even have a sheriff of sorts, who wasn't really expanded upon. When Juliet, even, is pardoned by Ben after she shot Danny, she still receives marking (branding burnt into her back) and disassociation.

What I don't understand is how Ben can hold Widmore as still somehow beholden to rules considering he was banished by this society, and didn't even pull the trigger regardless. Ben talking of "rules" is essentially just him making an excuse in his mind to evade responsibility for his failed bluff. He should have just surrendered. After all, if The Island has an ultimate destiny for him in store, which it does, then to surrender to Keamy should cause no fear in him at all. Ben was supposed to turn the donkey wheel. That was his path. Him being fearful of capture indicates a crisis of faith.

Martin Keamy is the villain of Season 4. We never see Charles ordering murder and carnage. He simply hired mercenary thugs who are overcome by their violent tendencies. But we also see that everyone else from the freighter was far more levelheaded. Miles Straume was even being paid $1.6 million just for providing a simple paranormal service. Widmore's mission is to target and extradite Benjamin Linus, retain his position of leadership, place known time-travelers in the vicinity to fulfill their historical purpose, and leave Jacob's candidates (mostly Oceanic castaways) to find their own destiny. He's not a moral man, but you'd be hard-pressed to convince anyone he was any worse than Linus.