Here is my wide-ranging (and that means long-winded, sorry guys. I'll give you a cookiee if you make it to the end ) theory on the Others :
The Others, as a whole, aren't part of the DHARMA Initiative. However, what they have done is taken over DHARMA's facilities on the island for their own purposes (as yet unknown, likely using the island's special properties) and even possibly converting some of their personnel without the DHARMA Initiative's knowledge.
As far as the Hanso Foundation (and associated companies, possibly including Widmore Industries) in the outside world is concerned, its island facilities are under attack by 'hostiles', as the Others have portrayed themselves : primitive, unsophisticated but deadly. Certainly, far from a corporate-backed ruthless and sophisticated organisation, which the Others likely are (it's possible at this time that Thomas Mittelwark, who is slowly increasing his influence within Hanso Foundation is one of Them, but as the Lost Experience is set a good two years after the crash, its hard to tell whether he has taken Alvar Hanso hostage and seized power yet at this stage).
It's important for the Others to maintain the illusion that DHARMA still controls the island, hence Mikhail (who may in fact be a converted DHARMA operative) wears a DHARMA uniform and continues to order food drops for the Swan station, which is manned by genuine DHARMA technicians, Calvin and Radsinksy. It's possible that Mikhail orders all the Others supplies through DHARMA, hence the importance of the Hanso Foundation not discovering the full extent to which their island has been compromised.
It is this imperative that affects the Others and all their dealings both on and off the island. The need to keep their presence (and the nature of that presence) a secret means they have limited submarine travel and most of their people must thus remain on the island indefinitely. This problem presents a recruitment issue and a control issue for those who are on the island but don't particularly like the idea of remaining indefinitely (e.g. Juliet). With limited resources, the senior Others have to play nice or risk a rebellion (hence always pretending to be the 'good guys' and Ben's obsession with his 'honest' image) but also be firm in keeping them there.
The Others didn't crash the plane, that was a result of a series of flukes ; flukes that were almost certainly influenced by the Island. By this stage, it's fairly clear that the Island is a seperate intelligence capable of communicating with humans. Ben thought there "might actually be" survivors, he wasn't counting on there been a large number of survivors (evidence that the Island, or the powered-up Electro-Magnet, healed many of them). Even though they are highly unlikely to escape, there is the possibility that one day someone will get off the island and expose their operation (any story they told would send off alarm bells in Hanso's ears). Thus, the Others keep their dealings with the Losties limited and try to hide their real nature. The senior Others can't simply kill them all (that would be too ruthless a gesture to keep up their nice facade) and even take it as an opportunity to recruit some of them.
In recruiting, the criteria are for 'good people' ; this is likely to help bolster the 'good guy' image but also would keep out violent or unstable newbies who could potentially rebel. Ideally, candidates would want to remain on the island and be malleable (read : controllable) in personality. For this reason, children are the perfect recruits, the younger the better. To this end, Ethan and Goodwin were sent in to form Lists, unaware of the size of the groups they'd be dealing with.
The first attack on the Tailies appeared strategic in removing the best able-bodied defenders. It's possible, due to their initial research, that the Others had already determined that these men were unworthy (the only one targeted who we know, Eko, had a blood-soaked past) and it's highly likely the three taken at this time were killed. The second took nine people on Goodwin's List, and Ana Lucia's killing of an Other was likely what delayed the capture of Cindy til the third attack. Also note that Goodwin considered Nathan to be "not a good person" and killed him when it suited his purposes. He wanted Ana Lucia to come, but his fellow Others vetoed it (she has a murder in her past).
Ethan, on the other hand, didn't complete a List. His task was complicated by Claire's pregnancy, an experiment which took priority over recruitment. Though he likely chalked up his discovery to Claire remembering the injections he administered, it's important to note that Claire had a dream in which she saw the cot where the Others wanted to put Aaron ; the Island (appearing as Locke) warned her and ruined Ethan's plan, exposing him. With the experiment unfinished, he grabbed Claire and Charlie, using the latter as a distraction for his pursuers. Either Charlie's drug addiction was enough to make him "bad", or Claire took absolute first priority over candidate lives. Or Charlie's done something we don't know about.
Continued on the next post, congratulations on making it this far !