The new guy's a scientisty guy, studying the zombies for cures or what advantages they can get... He's been used as an example of someone in the town that's not all bad, and that it's as conflicted a group as the main group is/was. At least that was how things were said online about him on the WD site.
I thought the killing of the men was brutal and somehow, in this whole mess, it's been one of the harder sequences for me to watch. I mean, I was pissed watching it which is silly but it still evoked that from me.
I think finding Darryl is just Merle's personal journey... Think he honestly cares for/about him. That'll play out interestingly. And to that end I think he wants to know where the farm was for A) supply purposes B) to maybe pick up his brothers trail, and C) he wants revenge on anyone who MAY have lived in the group that he didn't like. He seemed to honestly not mind Andrea and, like everyone else, Merle's another conflicted guy who doesn't hate everyone, just most everyone, and he's got his own morality.
This is one of the deeper series on TV I think, and really fascinates me. I'm now really curious who Carol Watcher was/is.

It's surely a threat, whoever it is.
This show gets better every week.
I thought Michonne's acting was a bit meh though.
Oh, and I am guessing they like Andrea and Michonne because they seem competent, useful people, plus they're women and women would be almost like another "supply" to some men, as society degenerated like it has. Again I think that's why the men get killed... they're a threat, and a drain on resources, when they have a "team" to already do that.
As a soldier unit they'd be allied to one another, closely. They're all trained physically and with weapons. And they're less useful than a couple women you think you can control and manipulate.